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Title: Seaside Studies in Natural History - Marine Animals of Massachusetts Bay. Radiates.
Author: Agassiz, Elizabeth Cabot Cary, 1822-1907, Agassiz, Alexander
Language: English
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SEASIDE STUDIES
IN
NATURAL HISTORY.
BY
ELIZABETH C. AGASSIZ
AND
ALEXANDER AGASSIZ.
MARINE ANIMALS OF MASSACHUSETTS BAY.
RADIATES.
[Illustration]
BOSTON:
JAMES R. OSGOOD AND COMPANY,
LATE TICKNOR & FIELDS, AND FIELDS, OSGOOD, & CO.
1871.
Entered according to Act of Congress, in the year 1865, by
A L E X A N D E R A G A S S I Z,
in the Clerk's Office of the District Court
for the District of Massachusetts.
UNIVERSITY PRESS:
WELCH, BIGELOW, AND COMPANY,
CAMBRIDGE.
THIS LITTLE BOOK
IS AFFECTIONATELY DEDICATED BY THE AUTHORS TO
PROFESSOR L. AGASSIZ,
WHOSE PRINCIPLES OF CLASSIFICATION HAVE BEEN THE MAIN
GUIDE IN ITS PREPARATION.
* * * * *
PREFACE.
This volume is published with the hope of supplying a want often
expressed for some seaside book of a popular character, describing the
marine animals common to our shores. There are many English books of
this kind; but they relate chiefly to the animals of Great Britain,
and can only have a general bearing on those of our own coast, which
are for the most part specifically different from their European
relatives. While keeping this object in view, an attempt has also been
made to present the facts in such a connection, with reference to
principles of science and to classification, as will give it in some
sort the character of a manual of Natural History, in the hope of
making it useful not only to the general reader, but also to teachers
and to persons desirous of obtaining a more intimate knowledge of the
subjects discussed in it. With this purpose, although nearly all the
illustrations are taken from among the most common inhabitants of our
bay, a few have been added from other localities in order to fill out
this little sketch of Radiates, and render it, as far as is possible
within such limits, a complete picture of the type.
A few words of explanation are necessary with reference to the joint
authorship of the book. The drawings and the investigations, where
they are not referred to other observers, have been made by MR. A.
AGASSIZ, the illustrations having been taken, with very few
exceptions, from nature, in order to represent the animals, as far as
possible, in their natural attitudes; and the text has been written by
MRS. L. AGASSIZ, with the assistance of MR. A. AGASSIZ's notes and
explanations.
CAMBRIDGE, May, 1865.
* * * * *
NOTE.
This second edition is a mere reprint of the first. A few mistakes
accidentally overlooked have been corrected; an explanation of the
abbreviations of the names of writers used after the scientific names
has been added, as well as a list of the wood-cuts. The changes which
have taken place in the opinions of scientific men with regard to the
distribution of animal life in the ocean have been duly noticed in
their appropriate place, but no attempt has been made to incorporate
more important additions which the progress of our knowledge of
Radiates may require hereafter.
CAMBRIDGE, January, 1871.
* * * * *
CONTENTS.
PAGE
ON RADIATES IN GENERAL 1
GENERAL SKETCH OF THE POLYPS 5
ACTINOIDS 7
MADREPORIANS 16
HALCYONOIDS 19
GENERAL SKETCH OF ACALEPHS 21
CTENOPHORÆ 26
EMBRYOLOGY OF CTENOPHORÆ 34
DISCOPHORÆ 37
HYDROIDS 49
MODE OF CATCHING JELLY-FISHES 85
ECHINODERMS 91
HOLOTHURIANS 95
ECHINOIDS 101
STAR-FISHES 108
OPHIURANS 115
CRINOIDS 120
EMBRYOLOGY OF ECHINODERMS 123
DISTRIBUTION OF LIFE IN THE OCEAN 141
SYSTEMATIC TABLE 152
INDEX 154
LIST OF THE WOOD-CUTS.
Unless otherwise specified, the illustrations are drawn from nature
by ALEX. AGASSIZ.
FIG. PAGE
1. Transverse section of an Actinia (Agassiz) 5
2, 3, 4. Actinia in different degrees of expansion (Agassiz) 8
5. METRIDIUM MARGINATUM fully expanded 8
6. Vertical section of an Actinia 10
7. View from above of an expanded Actinia 11
8, 9. Young Actiniæ 11
10. RHODACTINIA DAVISII 13
11. ARACHNACTIS BRACHIOLATA 14
12. Young Arachnactis 14
13. Young Arachnactis showing the mouth 14
14. BICIDIUM PARASITICUM 15
15. HALCAMPA ALBIDA 16
16. Colony of ASTRANGIA DANÆ 17
17. Magnified individuals of Astrangia 17
18. Single individual of Astrangia 18
19. Lasso-cell of Astrangia 18
20. Limestone pit of Astrangia 19
21. Single individual of HALCYONIUM CARNEUM 19
22. Halcyonium community 20
23. Expanded individual of Halcyonium 20
24. Branch of MILLEPORA ALCICORNIS (Agassiz) 22
25. Expanded animals of Millepora (Agassiz) 22
26. Transverse section of branch of Millepora (Agassiz) 23
27. PLEUROBRACHIA RHODODACTYLA (Agassiz) 27
28. The same as Fig. 27 seen in plane of tentacles (Agassiz) 28
29. Pleurobrachia in motion 29
30. Pleurobrachia seen from the extremity opposite the mouth 30
31. BOLINA ALATA seen from the broad side (Agassiz) 31
32. Bolina seen from the narrow side (Agassiz) 31
33. IDYIA ROSEOLA seen from the broad side (Agassiz) 32
34. Young Pleurobrachia still in the egg 35
35. Young Pleurobrachia swimming in the egg 35
36. Young Pleurobrachia resembling already adult 35
37. Young Idyia 35
38. Young Idyia seen from the anal pole 36
39. Idyia somewhat older than Fig. 37 36
40. Idyia still older 36
41. Young Bolina in stage resembling Pleurobrachia 37
42. Young Bolina seen from the broad side 37
43. Young Bolina seen from the narrow side 37
44. CYANEA ARCTICA 40
45. Scyphistoma of Aurelia (Agassiz) 41
46. Scyphistoma older than Fig. 45 (Agassiz) 41
47. Strobila of Aurelia (Agassiz) 41
48. Ephyra of Aurelia (Agassiz) 42
49. AURELIA FLAVIDULA seen in profile (Agassiz) 42
50. Aurelia seen from above (Agassiz) 43
51. CAMPANELLA PACHYDERMA 44
52. The same from below 44
53. TRACHYNEMA DIGITALE 45
54. HALICLYSTUS AURICULA 46
55. Lucernaria seen from the mouth side 47
56. Young Lucernaria 48
57. Hydrarium of EUCOPE DIAPHANA 50
58. Magnified portion of Fig. 57 50
59. Part of marginal tentacles of Eucope 51
60. Young Eucope 51
61. Adult Eucope, profile 51
62. Quarter-disk of Fig. 60 51
63. Quarter-disk of Eucope older than Fig. 62 52
64. Quarter-disk of adult Eucope 52
65. OCEANIA LANGUIDA just escaped from the reproductive calycle 53
66. Same as Fig. 65 from below 53
67. Young Oceania older than Fig. 65 54
Diagram of succession of tentacles 54
68. Adult Oceania 55
69. Attitude assumed by Oceania 56
70. CLYTIA BICOPHORA escaped from reproductive calycle 57
71. Somewhat older than Fig. 70 57
72. Magnified portion of Hydrarium of Clytia 57
73. Adult Clytia 57
74. ZYGODACTYLA GROENLANDICA 58
75. The same seen in profile 59
76. TIMA FORMOSA 61
77. One of the lips of the mouth 61
78. Head of Hydrarium of Tima 62
79. MELICERTUM CAMPANULA from above (Agassiz) 63
80. The same seen in profile 64
81. Planula of Melicertum 65
82. Cluster of planulæ 65
83. Young Hydrarium 65
84. DYNAMENA PUMILA 66
85. Magnified portion of Fig. 84 66
86. DYPHASIA ROSACEA 67
87. Medusa of LAFOEA 67
88. Colony of Coryne mirabilis (Agassiz) 68
89. Magnified head of Fig. 88 (Agassiz) 68
90. Free Medusa of Coryne (Agassiz) 68
91. TURRIS VESICARIA 69
92. BOUGAINVILLIA SUPERCILIARIS 70
93. Hydrarium of Bougainvillia 70
94, 95, 96. Medusæ buds of Fig. 93 71
97. Young Medusa just freed from the Hydroid 71
98. TUBULARIA COUTHOUYI (Agassiz) 72
99. Cluster of Medusæ of Fig. 98 (Agassiz) 72
100. Female colony of HYDRACTINIA POLYCLINA (Agassiz) 73
101. Male colony of the same (Agassiz) 73
102. Unsymmetrical Medusa of HYBOCODON PROLIFER (Agassiz) 74
103. Medusa bud of Hybocodon (Agassiz) 74
104. Hybocodon Hydrarium (Agassiz) 74
105. DYSMORPHOSA FULGURANS 75
106. Proboscis of Fig. 105 with young Medusæ 75
107. Young NANOMIA CARA 76
108. Nanomia with rudimentary Medusæ 76
109. Nanomia somewhat older than Fig. 108 77
110. Heart-shaped swimming bell of Nanomia 77
111. Cluster of Medusæ with tentacles having pendent knobs 78
112. Magnified pendent knob 79
113. Medusa with corkscrew-shaped tentacles 79
114. Medusa with simple tentacle 80
115. Adult Nanomia 81
116. Oil float of Nanomia 82
117. PHYSALIA ARETHUSA (Agassiz) 83
118. Bunch of Hydræ (Agassiz) 84
119. Cluster of Medusæ (Agassiz) 86
120. VELELLA MUTICA (Agassiz) 84
121. Free Medusa of Velella (Agassiz) 84
122. PTYCHOGENA LACTEA 86
123. Ovary of Ptychogena 87
124. SYNAPTA TENUIS 95
125. Anchor of Synapta 96
126. CAUDINA ARENATA 97
127. CUVIERIA SQUAMATA 98
128. Young Cuvieria 99
129. Cuvieria somewhat older than Fig. 128 99
130. PENTACTA FRONDOSA 100
131. TOXOPNEUSTES DROBACHIENSIS 102
132. Portion of shell of Fig. 131 without spines (Agassiz) 103
133. Sea-urchin shell without spines (Agassiz) 103
134. Sea-urchin from the mouth side (Agassiz) 104
135. Magnified spine 104
136. Transverse section of spine 105
137. Pedicellaria of Sea-urchin 105
138. Teeth of Sea-urchin 106
139. ECHINARACHNIUS PARMA 107
140. Transverse section of Echinarachnius (Agassiz) 108
141. Ray of Star-fish, seen from mouth side (Agassiz) 109
142. ASTRACANTHION BERYLINUS 110
143. Single spine of Star-fish 111
144. Limestone network of back of Star-fish 111
145. Madreporic body of Star-fish 111
146. CRIBRELLA OCULATA 112
147. CTENODISCUS CRISPATUS 114
148. OPHIOPHOLIS BELLIS 115
149. Arm of Fig. 148, from the mouth side (Agassiz) 116
150. Tentacle of Ophiopolis 116
151. ASTROPHYTON AGASSIZII 118
152. Pentacrinus 121
153. ALECTO MERIDIONALIS 122
154. Young Comatulæ 122
155, 156, 157. Egg of Star-fish in different stages
of development 124
158. Larva just hatched from egg 125
159-164. Successive stages of development of Larva 125
165. Larva in which arms are developing 126
166. Adult Star-fish Larva (BRACHIOLARIA) 127
167. Fig. 166 seen in profile 128
168-170. Young Star-fish (Astracanthion) in different stages
of development 129
171. Lower side of ray of young Star-fish 130
172. Very young Star-fish seen in profile 130
173-175. Larvæ of Sea-urchin (Toxopneustes) in different stages
of development 130, 131
176. Adult Larva of Sea-urchin 132
177. Fig. 176 seen endways 133
178. Sea-urchin resorbing the arms of the larva 133
179-181. Successive stages of young Sea-urchin 133, 134
182. Ophiuran which has nearly resorbed the larva 135
183. Larva of Ophiuran (Pluteus) 136
184. Young Ophiuran 137
185. Cluster of eggs of Star-fishes over mouth of parent 137
Diagram of a rocky beach 149
ABBREVIATIONS OF THE NAMES OF AUTHORS.
AG. L. Agassiz.
A. AG. A. Agassiz.
AYRES W. O. Ayres.
BLAINV. Blainville.
BOSC Bosc.
BR. Brandt.
CLARK H. J. Clark.
CUV. Cuvier.
D. & K. Düben and Koren.
EDW. Milne-Edwards.
FORBES Edw. Forbes.
GRAY J. E. Gray.
JAEG. Jaeger.
LAM. Lamarck.
LAMX. Lamouroux.
LIN. Linnæus.
LYM. Lyman.
M. & T. Müller and Troschel.
MILL. Miller.
PÉR. et LES. Péron and Lesueur.
SARS M. Sars.
STIMP. Stimpson.
TIL. Tilesius.
* * * * *
MARINE ANIMALS OF MASSACHUSETTS BAY.
ON RADIATES IN GENERAL.
It is perhaps not strange that the Radiates, a type of animals whose
home is in the sea, many of whom are so diminutive in size, and so
light and evanescent in substance, that they are hardly to be
distinguished from the element in which they live, should have been
among the last to attract the attention of naturalists. Neither is it
surprising to those who know something of the history of these
animals, that when the investigation of their structure was once
begun, when some insight was gained into their complex life, their
association in fixed or floating communities, their wonderful
processes of development uniting the most dissimilar individuals in
one and the same cycle of growth, their study should have become one
of the most fascinating pursuits of modern science, and have engaged
the attention of some of the most original investigators during the
last half century. It is true that from the earliest days of Natural
History, the more conspicuous and easily accessible of these animals
attracted notice and found their way into the scientific works of the
time. Even Aristotle describes some of them under the names of
Acalephæ and Knidæ, and later observers have added something, here and
there, to our knowledge on the subject; but it is only within the last
fifty years that their complicated history has been unravelled, and
the facts concerning them presented in their true connection.
Among the earlier writers on this subject we are most indebted to
Rondelet, in the sixteenth century, who includes some account of the
Radiates, in his work on the marine animals of the Mediterranean. His
position as Professor in the University at Montpelier gave him an
admirable opportunity, of which he availed himself to the utmost, for
carrying out his investigations in this direction. Seba and Klein, two
naturalists in the North of Europe, also published at about this time
numerous illustrations of marine animals, including Radiates. But in
all these works we find only drawings and descriptions of the animals,
without any attempt to classify them according to common structural
features. In 1776, O. F. Müller, in a work on the marine and
terrestrial faunæ of Denmark, gave some admirable figures of Radiates,
several of which are identical with those found on our own coast.
Cavolini also in his investigations on the lower marine animals of the
Mediterranean, and Ellis in his work upon those of the British coast,
did much during the latter half of the past century to enlarge our
knowledge of them.
It was Cuvier, however, who first gave coherence and precision to all
previous investigations upon this subject, by showing that these
animals are united on a common plan of structure expressively
designated by him under the name Radiata. Although, from a mistaken
appreciation of their affinities, he associated some animals with them
which do not belong to the type, and have since, upon a more intimate
knowledge of their structure, been removed to their true positions;
yet the principle introduced by him into their classification, as well
as into that of the other types of the animal kingdom, has been all
important to science.
It was in the early part of this century that the French began to
associate scientific objects with their government expeditions.
Scarcely any important voyage was undertaken to foreign countries by
the French navy which did not include its corps of naturalists, under
the patronage of government. Among the most beautiful figures we have
of Radiates, are those made by Savigny, one of the French naturalists
who accompanied Napoleon to Egypt; and from this time the lower marine
animals began to be extensively collected and studied in their living
condition. Henceforth the number of investigators in the field became
more numerous, and it may not be amiss to give here a slight account
of the more prominent among them.
Darwin's fascinating book, published after his voyage to the Pacific,
and giving an account of the Coral islands, the many memoirs of Milne
Edwards and Haime, and the great works of Quoy and Gaimard, and of
Dana, are the chief authorities upon Polyps. In the study of the
European Acalephs we have a long list of names high in the annals of
science. Eschscholtz, Péron and Lesueur, Quoy and Gaimard, Lesson,
Mertens, and Huxley, have all added largely to our information
respecting these animals, their various voyages having enabled them to
extend their investigations over a wide field. No less valuable have
been the memoirs of Kölliker, Leuckart, Gegenbaur, Vogt, and Haeckel,
who in their frequent excursions to the coasts of Italy and France
have made a special study of the Acalephs, and whose descriptions have
all the vividness and freshness which nothing but familiarity with the
living specimens can give. Besides these, we have the admirable works
of Von Siebold, of Ehrenberg, the great interpreter of the microscopic
world, of Steenstrup, Dujardin, Dalyell, Forbes, Allman, and Sars. Of
these, the four latter were fortunate in having their home on the
sea-shore within reach of the objects of their study, so that they
could watch them in their living condition, and follow all their
changes. The charming books of Forbes, who knew so well how to
popularize his instructions, and present scientific results under the
most attractive form, are well known to English readers. But a word on
the investigations of Sars may not be superfluous.
Born near the coast of Norway, and in early life associated with the
Church, his passion for Natural History led him to employ all his
spare time in the study of the marine animals immediately about him,
and his first papers on this subject attracted so much attention, that
he was offered the place of Professor at Christiania, and henceforth
devoted himself exclusively to scientific pursuits, and especially to
the investigation of the Acalephs. He gave us the key to the almost
fabulous transformations of these animals, and opened a new path in
science by showing the singular phenomenon of the so-called "alternate
generations," in which the different phases of the same life may be so
distinct and seemingly so disconnected that, until we find the
relation between them, we seem to have several animals where we have
but one.
To the works above mentioned, we may add the third and fourth volumes
of Professor Agassiz's Contributions to the Natural History of the
United States, which are entirely devoted to the American Acalephs.
The most important works and memoirs concerning the Echinoderms are
those by Klein, Link, Johannes Müller, Jäger, Desmoulins, Troschel,
Sars, Savigny, Forbes, Agassiz, and Lütken, but excepting those of
Forbes and Sars, few of these observations are made upon the living
specimens. It may be well to mention here, for the benefit of those
who care to know something more of the literature of this subject in
our own country, a number of memoirs on the Radiates of our coasts,
published by the various scientific societies of the United States,
and to be found in their annals. Such are the papers of Gould,
Agassiz, Leidy, Stimpson, Ayres, McCrady, Clark, A. Agassiz, and
Verrill.
One additional word as to the manner in which the subjects included in
the following descriptions are arranged. We have seen that Cuvier
recognized the unity of plan in the structure of the whole type of
Radiates. All these animals have their parts disposed around a common
central axis, and diverging from it toward the periphery. The idea of
bilateral symmetry, or the arrangement of parts on either side of a
longitudinal axis, on which all the higher animals are built, does not
enter into their structure, except in a very subordinate manner,
hardly to be perceived by any but the professional naturalist. This
radiate structure being then common to the whole type, the animals
composing it appear under three distinct structural expressions of the
general plan, and according to these differences are divided into
three classes,--Polyps, Acalephs, and Echinoderms. With these few
preliminary remarks we may now take up in turn these different groups,
beginning with the lowest, or the Polyps.[1]
[Footnote 1: It is to be regretted that on account of the meagre
representations of Polyps on our coast, where the coral reefs, which
include the most interesting features of Polyp life, are entirely
wanting, our account of these animals is necessarily deficient in
variety of material. When we reach the Acalephs or Jelly-Fishes, in
which the fauna of our shores is especially rich, we shall not have the
same apology for dulness; and it will be our own fault if our readers
are not attracted by the many graceful forms to which we shall then
introduce them.]
* * * * *
GENERAL SKETCH OF THE POLYPS.
Before describing the different kinds of Polyps living on our
immediate coast, we will say a few words of Polyps in general and of
the mode in which the structural plan common to all Radiates is
adapted to this particular class. In all Polyps the body consists of a
sac divided by vertical partitions (Fig. 1.) into distinct cavities or
chambers. These partitions are not, however, all formed at once, but
are usually limited to six at first, multiplying indefinitely with the
growth of the animal in some kinds, while in others they never
increase beyond a certain definite number. In the axis of the sac,
thus divided, hangs a smaller one, forming the digestive cavity, and
supported for its whole length by the six primary partitions. The
other partitions, though they extend more or less inward in proportion
to their age, do not unite with the digestive sac, but leave a free
space in the centre between their inner edge and the outer wall of the
digestive sac. The genital organs are placed on the inner edges of the
partitions, thus hanging as it were at the door of the chambers, so
that when hatched, the eggs naturally drop into the main cavity of the
body, whence they pass into the second smaller sac through an opening
in its bottom or digestive cavity, and thence out through the mouth
into the water. In the lower Polyps, as in our common Actinia for
instance, these organs occur on all the radiating partitions, while
among the higher ones, the Halcyonoids for example, they are found
only on a limited number. This limitation in the repetition of
identical parts is always found to be connected with structural
superiority.
[Illustration: Fig. 1. Transverse section of an Actinia.
(_Agassiz_.)]
The upper margin of the body is fringed by hollow tentacles, each of
which opens into one of the chambers. All parts of the animal thus
communicate with each other, whatever is introduced at the mouth
circulating through the whole structure, passing first into the
digestive cavity, thence through the opening in the bottom into the
main chambered cavity, where it enters freely into all the chambers,
and from the chambers into the tentacles. The rejected portions of the
food, after the process of digestion is completed, return by the same
road and are thrown out at the mouth.
These general features exist in all Polyps, and whether they lead an
independent life as the Actinia, or are combined in communities, like
most of the corals and the Halcyonoids; whether the tentacles are many
or few; whether the partitions extend to a greater or less height in the
body; whether they contain limestone deposit, as in the corals, or
remain soft throughout life as the sea-anemone,--the above description
applies to them all, while the minor differences, either in the
tentacles or in the form, size, color, and texture of the body, are
simply modifications of this structure, introducing an infinite variety
into the class, and breaking it up into the lesser groups designated as
orders, families, genera, and species. Let us now look at some of the
divisions thus established.
The class of Polyps is divided into three orders,--the Halcyonoids,
the Madreporians, and the Actinoids. Of the lowest among these orders,
the Actinoid Polyps, our Actinia or sea-anemone is a good example.
They remain soft through life, having a great number of partitions and
consequently a great number of tentacles, since there is a tentacle
corresponding to every chamber. Indeed, in this order the
multiplication of tentacles and partitions is indefinite, increasing
during the whole life of the animal with its growth; while we shall
see that in some of the higher orders the constancy and limitation in
the number of these parts is an indication of superiority, being
accompanied by a more marked individualization of the different
functions.
Next come the Madreporians, of which our Astrangia, to be described
hereafter, may be cited as an example. In this group, although the
number of tentacles still continues to be large, they are nevertheless
more limited than in the Actinoids; but their characteristic feature
is the deposition of limestone walls in the centre of the chambers
formed by the soft partitions, so that all the soft partitions
alternate with hard ones. The tentacles, always corresponding to the
cavity of the chambers, may be therefore said to ride this second set
of partitions arising just in the centre of the chambers.
The third and highest order of Polyps is that of the Halcyonoids. Here
the partitions are reduced to eight; the tentacles, according to the
invariable rule, agree in number with the chambers, but have a far
more highly complicated structure than in the lower Polyps. Some of
these Halcyonoids deposit limestone particles in their frame. But the
tendency to solidify is not limited to definite points, as in the
Madreporians. It may take place anywhere, the rigidity of the whole
structure increasing of course in proportion to the accumulation of
limestone. There are many kinds, in which the axis always remains soft
or cartilaginous, while others, as the so-called sea-fans for
instance, well known among the corals for their beauty of form and
color, are stiff and hard throughout. Whatever their character in this
respect, however, they are always compound, living in communities, and
never found as separate individuals after their early stages of
growth. Some of those with soft axis lead a wandering life, enjoying
as much freedom of movement as if they had an individual existence,
shooting through the water like the Pennatulæ, well known on the
California coast, or working their way through the sand like the
Renilla, common on the sandy shores of our Southern States.
* * * * *
ACTINOIDS.
_Actinia, or Sea-Anemone_. (_Metridium marginatum_ EDW.)
Nothing can be more unprepossessing than a sea-anemone when
contracted. A mere lump of brown or whitish jelly, it lies like a
lifeless thing on the rock to which it clings, and it is difficult to
believe that it has an elaborate and exceedingly delicate internal
organization, or will ever expand into such grace and beauty as really
to deserve the name of the flower after which it has been called.
Figs. 2, 3, 4, and 5, show this animal in its various stages of
expansion and contraction. Fig. 2 represents it with all its external
appendages folded in, and the whole body flattened; in Fig. 3, the
tentacles begin to steal out, and the body rises slightly; in Fig. 4,
the body has nearly gained its full height, and the tentacles, though
by no means fully spread, yet form a delicate wreath around the mouth;
while in Fig. 5, drawn in life size, the whole summit of the body
seems crowned with soft, plumy fringes. We would say for the benefit
of collectors that these animals are by no means difficult to find,
and thrive well in confinement, though it will not do to keep them in
a small aquarium with other specimens, because they soon render the
water foul and unfit for their companions. They should therefore be
kept in a separate glass jar or bowl, and under such circumstances
will live for a long time with comparatively little care.
[Illustration: Figs. 2, 3, 4. Actinia in different degrees of
expansion. (_Agassiz_.)]
[Illustration: Fig. 5. The same Actinia (Metridium marginatum)
fully expanded; natural size.]
They may be found in any small pools about the rocks which are flooded
by the tide at high water. Their favorite haunts, however, where they
occur in greatest quantity are more difficult to reach; but the
curious in such matters will be well rewarded, even at the risk of wet
feet and a slippery scramble over rocks covered with damp sea-weed, by
a glimpse into their more crowded abodes. Such a grotto is to be found
on the rocks of East Point at Nahant. It can only be reached at low
tide, and then one is obliged to creep on hands and knees to its
entrance, in order to see through its entire length; but its whole
interior is studded with these animals, and as they are of various
hues, pink, brown, orange, purple, or pure white, the effect is like
that of brightly colored mosaics set in the roof and walls. When the
sun strikes through from the opposite extremity of this grotto, which
is open at both ends, lighting up its living mosaic work, and showing
the play of the soft fringes wherever the animals are open, it would
be difficult to find any artificial grotto to compare with it in
beauty. There is another of the same kind on Saunders's Ledge, formed
by a large boulder resting on two rocky ledges, leaving a little cave
beneath, lined in the same way with variously colored sea-anemones, so
closely studded over its walls that the surface of the rock is
completely hidden. They are, however, to be found in larger or smaller
clusters, or scattered singly in any rocky fissures, overhung by
sea-weed, and accessible to the tide at high water.
The description of Polyp structure given above includes all the
general features of the sea-anemone; but for the better explanation of
the figures, it may not be amiss to recapitulate them here in their
special application. The body of the sea-anemone may be described as a
circular, gelatinous bag, the bottom of which is flat and slightly
spreading around the margin. (Fig. 2.) The upper edge of this bag
turns in so as to form a sac within a sac. (Fig. 6.) This inner sac,
_s_, is the stomach or digestive cavity, forming a simple open space
in the centre of the body, with an aperture in the bottom, _b_,
through which the food passes into the larger sac, in which it is
enclosed. But this outer and larger sac or main cavity of the body is
not, like the inner one, a simple open space. It is, on the contrary,
divided by vertical partitions into a number of distinct chambers,
converging from the periphery to the centre. These partitions do not
all advance so far as actually to join the wall of the digestive
cavity hanging in the centre of the body, but most of them stop a
little short of it, leaving thus a small, open space between the
chambers and the inner sac. (Fig. 1.) The eggs hang on the inner edge
of the partitions; when mature they drop into the main cavity, enter
the inner digestive cavity through its lower opening, and are passed
out through the mouth.
[Illustration: Fig. 6. Vertical section of an Actinia, showing a
primary _(g)_ and a secondary partition of _g'_; _o_ mouth, _t_
tentacles, _s_ stomach, _f f_ reproductive organs, _b_ main
cavity, _c_ openings in partitions, _a_ lower floor, or foot.]
The embryo bears no resemblance to the mature animal. It is a little
planula, semi-transparent, oblong, entirely covered with vibratile
cilia, by means of which it swims freely about in the water till it
establishes itself on some rocky surface, the end by which it becomes
attached spreading slightly and fitting itself to the inequalities of
the rock so as to form a secure basis. The upper end then becomes
depressed toward the centre, that depression deepening more and more
till it forms the inner sac, or in other words the digestive cavity
described above. The open mouth of this inner sac, which may, however,
be closed at will, since the whole substance of the body is
exceedingly contractile, is the oral opening or so-called mouth of the
animal. We have seen how the main cavity becomes divided by radiating
partitions into numerous chambers; but while these internal changes
are going on, corresponding external appendages are forming in the
shape of the tentacles, which add so much to the beauty of the animal,
and play so important a part in its history. The tentacles, at first
only few in number, are in fact so many extensions of the inner
chambers, gradually narrowing upward till they form these delicate
hollow feelers which make a soft downy fringe all around the mouth.
(Fig. 7.) They do not start abruptly from the summit, but the upper
margin of the body itself thins out to form more or less extensive
lobes, through which the partitions and chambers continue their
course, and along the edge of which the tentacles arise.
[Illustration: Fig. 7. View from above of an Actinia with all its
tentacles expanded; _o_ mouth, _b_ crescent-shaped folds at
extremity of mouth, _a a_ folds round mouth, _t t t_ tentacles.]
[Illustration: Figs. 8, 9. Young Actiniæ in different stages of
growth.]
The eggs are not always laid in the condition of the simple planula
described above. They may, on the contrary, be dropped from the parent
in different stages of development, sometimes even after the tentacles
have begun to form, as in Figs. 8, 9. Neither is it by means of eggs
alone that these animals reproduce themselves; they may also multiply
by a process of self-division. The disk of an Actinia may contract
along its centre till the circular outline is changed to that of a
figure 8, this constriction deepening gradually till the two halves of
the 8 separate, and we have an Actinia with two mouths, each
surrounded by an independent set of tentacles. Presently this
separation descends vertically till the body is finally divided from
summit to base, and we have two Actiniæ where there was originally but
one. Another and a far more common mode of reproduction among these
animals is that of budding like corals. A slight swelling arises on
the side of the body or at its base; it enlarges gradually, a
digestive cavity is formed within it, tentacles arise around its
summit, and it finally drops off from the parent and leads an
independent existence. As a number of these buds are frequently formed
at once, such an Actinia, surrounded by its little family, still
attached to the parent, may appear for a time like a compound stock,
though their normal mode of existence is individual and distinct.
The Actinia is exceedingly sensitive, contracting the body and drawing
in the tentacles almost instantaneously at the slightest touch. These
sudden movements are produced by two powerful sets of muscles, running
at right angles with each other through the thickness of the body
wall; the one straight and vertical, extending from the base of the
wall to its summit; the other circular and horizontal, stretching
concentrically around it. By the contraction of the former, the body
is of course shortened; by the contraction of the latter, the body is,
on the contrary, lengthened in proportion to the compression of its
circumference. Both sets can easily be traced by the vertical and
horizontal lines crossing each other on the external wall of the body,
as in Fig. 5. Each tentacle is in like manner furnished with a double
set of muscles, having an action similar to that described above. In
consequence of these violent muscular contractions, the water imbibed
by the animal, and by which all its parts are distended to the utmost,
is forced, not only out of the mouth, but also through small openings
in the body wall scarcely perceptible under ordinary circumstances,
but at such times emitting little fountains in every direction.
Notwithstanding its extraordinary sensitiveness, the organs of the
senses in the Actinia are very inferior, consisting only of a few
pigment cells accumulated at the base of the tentacles. The two sets
of muscles meet at the base of the body, forming a disk, or kind of
foot, by which the animal can fix itself so firmly to the ground, that
it is very difficult to remove it without injury. It is nevertheless
capable of a very limited degree of motion, by means of the expansion
and contraction of this foot-like disk.
The Actiniæ are extremely voracious; they feed on mussels and cockles,
sucking the animals out of their shells. When in confinement they may
be fed on raw meat, and seem to relish it; but if compelled to do so,
they will live on more meagre fare, and will even thrive for a long
time on such food as they may pick up in the water where they are
kept.
_Rhodactinia_ (_Rhodactinia Davisii_ AG.)
[Illustration: Fig. 10. Rhodactinia Davisii Ag.; natural size.]
Very different from this is the bright red Rhodactinia (Fig. 10),
quite common in the deeper waters of our bay, while farther north, in
Maine, it occurs at low-water mark. Occasionally it may be found
thrown up on our sandy beaches after a storm, and then, if it has not
been too long out of its native element, or too severely buffeted by
the waves, it will revive on being thrown into a bucket of fresh
sea-water, expand to its full size, and show all the beauty of its
natural coloring. It is crowned with a wreath of thick, short
tentacles (Fig. 10), and though so vivid and bright in color, it is
not so pretty as the more common Actinia marginata, with its soft
waving wreath of plume-like feelers, in comparison to which the
tentacles of the Rhodactinia are clumsy and slow in their movements.
All Actiniæ are not attached to the soil like those described above,
nor do they all terminate in a muscular foot, some being pointed or
rounded at their extremity. Many are nomadic, wandering about at will
during their whole lifetime, others live buried in the sand or mud,
only extending their tentacles beyond the limits of the hole where
they make their home; while others again lead a parasitic life,
fastening themselves upon our larger jelly-fish, the Cyaneæ, though
one is at a loss to imagine what sustenance they can derive from
animals having so little solidity, and consisting so largely of water.
_Arachnactis_. (_Arachnactis brachiolata_ A. AG.)
[Illustration: Fig. 11. Arachnactis brachiolata A. Ag., greatly
magnified.]
[Illustration: Fig. 12. Young Arachnactis.]
[Illustration: Fig. 13. Young Arachnactis seen so as to show the
mouth.]
Among the nomadic Polyps is a small floating Actinia, called
Arachnactis, (Fig. 11,) from its resemblance to a spider. They are
found in great plenty floating about during the night, feeling their
way in every direction by means of their tentacles, which are large in
proportion to the size of the animal, few in number, and turned
downward when in their natural attitude. The partitions and the
digestive cavity enclosed between them are short, as will be seen in
Fig. 11, when compared to the general cavity of the body floating
balloon-like above them. Around the mouth is a second row of shorter
tentacles, better seen in a younger specimen (Fig. 12). This Actinia
differs from those described above, in having two of the sides
flattened, instead of being perfectly circular. Looked at from above
(as in Fig. 13) this difference in the diameters is very perceptible;
there is an evident tendency towards establishing a longitudinal axis.
In the sea-anemone, this disposition is only hinted at in the slightly
pointed folds or projections on opposite sides of the circle formed by
the mouth, which in the Arachnactis are so elongated as to produce a
somewhat narrow slit (see Fig. 13), instead of a circular opening. The
mouth is also a little out of centre, rather nearer one end of the
disk than the other. These facts are interesting, as showing that the
tendency towards establishing a balance of parts, as between an
anterior and posterior extremity, a right and left side, is not
forgotten in these lower animals, though their organization as a whole
is based upon an equality of parts, admitting neither of posterior and
anterior extremities, nor of right and left, nor of above and below,
in a structural sense. This animal also presents a seeming anomaly in
the mode of formation of the young tentacles, which always make their
appearance at the posterior extremity of the longitudinal axis, the
new ones being placed behind the older ones, instead of alternating
with them as in other Actiniæ.
_Bicidium_. (_Bicidium parasiticum_ AG.)
[Illustration: Fig. 14. Bicidium parasiticum; natural size.]
The Bicidium (Fig. 14), our parasitic Actinia, is to be sought for in
the mouth-folds of the Cyanea, our common large red Jelly-fish. In any
moderate-sized specimen of the latter from twelve to eighteen inches
in diameter, we shall be sure to find one or more of these parasites,
hidden away among the numerous folds of the mouth. The body is long
and tapering, having an aperture in the extremity, the whole animal
being like an elongated cone, strongly ribbed from apex to base. At
the base, viz. at the month end, are a few short, stout tentacles.
This Actinia is covered with innumerable little transverse wrinkles
(see Fig. 14), by means of which it fastens itself securely among the
fluted membranes around the mouth of the Jelly-fish. It will live a
considerable time in confinement, attaching itself, for its whole
length, to the vessel in which it is kept, and clinging quite firmly
if any attempt is made to remove it. The general color of the body is
violet or a brownish red, though the wrinkles give it a somewhat
mottled appearance. _Halcampa_. (_Halcampa albida_ AG.)
Strange to say, the Actiniæ, which live in the mud, are among the most
beautifully colored of these animals. They frequently prepare their
home with some care, lining their hole by means of the same secretions
which give their slimy surface to our common Actiniæ, and thus forming
a sort of tube, into which they retire when alarmed. But if
undisturbed, they may be seen at the open door of their house with
their many colored disk and mottled tentacles extending beyond the
aperture, and their mouth wide open, waiting for what the tide may
bring them. By the play of their tentacles, they can always produce a
current of water about the mouth, by means of which food passes into
the stomach. We have said, that these animals are very brightly
colored, but the little Halcampa (Fig. 15), belonging to our coast, is
not one of the brilliant ones. It is, on the contrary, a small,
insignificant Actinia, resembling a worm, as it burrows its way
through the sand. It is of a pale yellowish color, with whitish warts
on the surface.
[Illustration: Fig. 15. Halcampa albida; natural size.]
* * * * *
MADREPORIANS.
_Astrangia_. (_Astrangia Danæ_ AG.)
In Figure 16, we have the only species of coral growing so far north
as our latitude. Indeed, it hardly belongs in this volume, since we
have limited ourselves to the Radiates of Massachusetts Bay,--its
northernmost boundary being somewhat to the south of Massachusetts
Bay, about the shores of Long Island, and on the islands of Martha's
Vineyard Sound. But we introduce it here, though it is not included
under our title, because any account of the Radiates, from which so
important a group as that of the corals was excluded, would be very
incomplete.
[Illustration: Fig. 16. Astrangia colony; natural size.]
[Illustration: Fig. 17. Magnified individuals of an Astrangia
community in different stages of expansion.]
This pretty coral of our Northern waters is no reef-builder, and does
not extend farther south than the shores of North Carolina. It usually
establishes itself upon broken angular bits of rock, lying in
sheltered creeks and inlets, where the violent action of the open sea
is not felt. The presence of one of these little communities on a rock
may first be detected by what seems like a delicate white film over
the surface. This film is, however, broken up by a number of hard
calcareous deposits in very regular form (Fig. 20), circular in
outline, but divided by numerous partitions running from the outer
wall to the centre of every such circle, where they unite at a little
white spot formed by the mouth or oral opening. These circles
represent, and indeed are themselves the distinct individuals (Fig.
17) composing the community, and they look not unlike the star-shaped
pits on a coral head, formed by Astræans. Unlike the massive compact
kinds of coral, however, the individuals multiply by budding from the
base chiefly, never rising one above the other, but spreading over the
surface on which they have established themselves, a few additional
individuals arising between the older ones. In consequence of this
mode of growth, such a community, when it has attained any size, forms
a little white mound on the rock, higher in the centre, where the
older members have attained their whole height and solidity, and
thinning out toward the margin, where the younger ones may be just
beginning life, and hardly rise above the surface of the rock. These
communities rarely grow to be more than two or three inches in
diameter, and about quarter of an inch in height at the centre where
the individuals have reached their maximum size. When the animals are
fully expanded (Fig. 18), with all their tentacles spread, the surface
of every such mound becomes covered with downy white fringes, and what
seemed before a hard, calcareous mass upon the rock, changes to a soft
fleecy tuft, waving gently to and fro in the water. The tentacles are
thickly covered with small wart-like appendages, which, on
examination, prove to be clusters of lasso-cells, the terminal cluster
of the tentacle being quite prominent. These lasso-cells are very
formidable weapons, judging both from their appearance when magnified
(Fig. 19), and from the terrible effect of their bristling lash upon
any small crustacean, or worm, that may be so unfortunate as to come
within its reach.
[Illustration: Fig. 18. Single individual of Astrangia, fully
expanded.]
[Illustration: Fig. 19. Magnified lasso-cell of Astrangia.]
The description of the internal arrangement of parts in the Actinia
applies in every particular to these corals, with the exception of the
hard deposit in the lower part of the body. As in all the Polyps,
radiating partitions divide the main cavity of the body into distinct
separate chambers, and the tentacles increasing by multiples of six,
numbering six in the first set, six in the second, and twelve in the
third, are hollow, and open into the chambers. But the feature which
distinguishes them from the soft Actiniæ, and unites them with the
corals, requires a somewhat more accurate description. In each
individual, a hard deposit is formed (Fig. 20), beginning at the base
of every chamber, and rising from its floor to about one fifth the
height of the animal at its greatest extension. This lime deposit does
not, however, fill the chamber for its whole width, but rises as a
thin wall in its centre. (See Figs. 13, 17.) Thus between all the soft
partitions, in the middle of the chambers which separate them, low
limestone walls are gradually built up, uniting in a solid column in
the centre. These walls run parallel with the soft partitions,
although they do not rise to the same height, and they form the
radiating lines like stiff lamellæ, so conspicuous when all the soft
parts of the body are drawn in. The mouth of the Astrangia is oval,
and the partitions spread in a fan-shaped way, being somewhat shorter
at one side of the animal than on the other. The partitions extend
beyond the solid wall which unites them at the periphery, in
consequence of which, this wall is marked by faint vertical ribs.
[Illustration: Fig. 20. Limestone parts of an individual of
Astrangia; magnified.]
* * * * *
HALCYONOIDS.
_Halcyonium_. (_Halcyonium carneum_ AG.)
[Illustration: Fig. 21. Single individual of Halcyonium seen from
above; magnified.]
We come now to the Halcyonoids, represented in our waters by the
Halcyonium (Fig. 22). In the Halcyonoids, the highest group of Polyps,
the tentacles reach their greatest limitation, which, as above
mentioned, is found to be a mark of superiority, and, connected with
other structural features, places them at the head of their class. The
number of tentacles throughout this group is always eight. They are
very complicated (Fig. 21), in comparison with the tentacles of the
lower orders, being deeply lobed, and fringed around the margin. Our
Halcyonium communities (Fig. 22) usually live in deep water, attached
to dead shells, though they may occasionally be found growing at
low-water mark, but this is very rare. They have received a rather
lugubrious name from the fishermen, who call them "dead-men's
fingers," and indeed, when the animals are contracted, such a
community, with its short branches attached to the main stock, looks
not unlike the stump of a hand, with short, fat fingers. In such a
condition they are very ugly, the whole mass being somewhat gelatinous
in texture, and a dull, yellowish pink in color. But when the animals,
which are capable of great extension, are fully spread, as in Fig. 22,
such a polyp-stock has a mossy, tufted look, and is by no means an
unsightly object. When the individuals are entirely expanded, as in
Fig. 23, they become quite transparent, and their internal structure
can readily be seen through the walls of the body; we can then easily
distinguish the digestive cavity, supported for its whole length by
the eight radiating partitions, as well as the great size of the main
digestive cavity surrounding it. Notwithstanding the remarkable power
of contraction and dilatation in the animals themselves, the tentacles
are but slightly contractile. This kind of community increases
altogether by budding, the individual polyps remaining more or less
united, the tissues of the individuals becoming thicker by the
deposition of lime nodules, and thus forming a massive
semi-cartilaginous pulp, uniting the whole community. In the
neighborhood of Provincetown they are very plentiful, and are found
all along the shores of our Bay in deep water.
[Illustration: Fig. 22. Halcyonium community; natural size.]
[Illustration: Fig. 23. Individual of Halcyonium fully expanded;
magnified.]
GENERAL SKETCH OF ACALEPHS.
In the whole history of metamorphosis, that wonderful chapter in the
life of animals, there is nothing more strange or more interesting
than the transformations of the Acalephs. First, as little floating
planulæ or transparent spheres, covered with fine vibratile cilia, by
means of which they move with great rapidity, then as communities
fixed to the ground and increasing by budding like the corals, or
multiplying by self-division, and later as free-swimming Jelly-fishes,
many of them pass through phases which have long baffled the
investigations of naturalists, and have only recently been understood
in their true connection. Great progress has, however, been made
during this century in our knowledge of this class. Thanks to the
investigations of Sars, Dujardin, Steenstrup, Van Beneden, and many
others, we now have the key to their true relations, and transient
phases of growth, long believed to be the adult condition of distinct
animals, are recognized as parts in a cycle of development belonging
to one and the same life. As the class now stands, it includes three
orders, highest among which are the CTENOPHORÆ, so called on account
of their locomotive organs, consisting of minute flappers arranged in
vertical comb-like rows; next to these are the DISCOPHORÆ, with their
large gelatinous umbrella-like disks, commonly called Jelly-fishes,
Sun-fishes, or Sea-blubbers, and below these come the HYDROIDS,
embracing the most minute and most diversified of all these animals.
These orders are distinguished not only by their striking external
differences, but by their mode of development also. The Ctenophoræ
grow from eggs by a direct continuous process of development, without
undergoing any striking metamorphosis; the Discophoræ, with some few
exceptions, in which they develop like the Ctenophoræ from eggs, begin
life as a Hydra-like animal, the subsequent self-division of which
gives rise, by a singular process, presently to be described, to a
number of distinct Jelly-fishes; the Hydroids include all those
Acalephs which either pass the earlier stages of their existence as
little shrub-like communities, or remain in that condition through
life. These Hydroid stocks, as they are sometimes called, give rise to
buds; these buds are transformed into Jelly-fishes, which in some
instances break off when mature and swim away as free animals, while
in others they remain permanent members of the Hydroid stock, never
assuming a free mode of life. All these buds when mature, whether free
or fixed, lay eggs in their turn, from which a fresh stock arises to
renew this singular cycle of growth, known among naturalists as
"alternate generations."
[Illustration: Fig. 24. Branch of Millepora alcicornis; natural
size. (_Agassiz_.)]
[Illustration: Fig. 25. Animals of M. alcicornis expanded;
magnified. _a_ _a_ _a_ small Hydroid, _b_ larger Hydroid, _t_
tentacles, _m_ mouth. (_Agassiz_.)]
The Hydroids are not all attached to the ground,--some like the
Physalia (Portuguese man-of-war), or the Nanomia, that pretty floating
Hydroid of our own waters, move about with as much freedom as if they
enjoyed an individual independent existence. As all these orders have
their representatives on our coast, to be described hereafter in
detail, we need only allude here to their characteristic features. But
we must not leave unnoticed one very remarkable Hydroid Acaleph (Fig.
24), not found in our waters, and resembling the Polyps so much, that
it has long been associated with them. The Millepore is a coral, and
was therefore the more easily confounded with the Polyps, so large a
proportion of which build coral stocks; but a more minute
investigation of its structure (Figs. 25, 26) has recently shown that
it belongs with the Acalephs.[2] This discovery is the more important,
not only as explaining the true position of this animal in the Animal
Kingdom, but as proving also the presence of Acalephs in the earliest
periods of creation, since it refers a large number of fossil corals,
whose affinities with the millepores are well understood, to that
class, instead of to the class of Polyps with which they had hitherto
been associated. But for this we should have no positive evidence of
the existence of Acalephs in early geological periods, the gelatinous
texture of the ordinary Jelly-fishes making their preservation almost
impossible. It is not strange that the true nature of this animal
should have remained so long unexplained; for it is only by the soft
parts of the body, not of course preserved in the fossil condition,
that their relations to the Acalephs may be detected; and they are so
shy of approach, drawing their tentacles and the upper part of the
body into their limestone frame if disturbed, that it is not easy to
examine the living animal.
[Footnote 2: See "Methods of Study," by Prof. Agassiz.]
[Illustration: Fig. 26. Transverse section of a branch, showing
pits, _a_ _a_ _a_ _a_, of the large Hydroids with the horizontal
floors. (_Agassiz_.)]
The Millepore is very abundant on the Florida reefs. From the solid
base of the coral stock arise broad ridges, branching more or less
along the edges, the whole surface being covered by innumerable pores,
from which the diminutive animals project when expanded. (Fig. 25.)
The whole mass of the coral is porous, and the cavities occupied by
the Hydræ are sunk perpendicularly to the surface within the stock.
Seen in a transverse cut these tubular cavities are divided at
intervals by horizontal partitions (Fig. 26), extending straight
across the cavity from wall to wall, and closing it up entirely, the
animal occupying only the outer-most open space, and building a new
partition behind it as it rises in the process of growth. This
structure is totally different from that of the Madrepores, Astræans,
Porites, and indeed, from all the polyp corals which, like all Polyps,
have the vertical partitions running through the whole length of the
body, and more or less open from top to bottom.
The life of the Jelly-fishes, with the exception of the Millepores and
the like, is short in comparison to that of other Radiates. While
Polyps live for many years, and Star-fishes and Sea-urchins require
ten or fifteen years to attain their full size, the short existence of
the Acaleph, with all its changes, is accomplished in one year. The
breeding season being in the autumn, the egg grows into a Hydroid
during the winter; in the spring the Jelly-fish is freed from the
Hydroid stock, or developed upon it as the case may be; it attains its
full size in the fall, lays its eggs and dies, and the cycle is
complete. The autumn storms make fearful havoc among them, swarms of
them being killed by the fall rains, after which they may be found
thrown up on the beaches in great numbers. When we consider the size
of these Jelly-fishes, their rapidity of growth seems very remarkable.
Our common Aurelia measures some twelve to eighteen inches in diameter
when full grown, and yet in the winter it is a Hydra so small as
almost to escape notice. Still more striking is the rapid increase of
our Cyanea, that giant among Jelly-fishes, which, were it not for the
soft, gelatinous consistency of its body, would be one of the most
formidable among our marine animals.
Before entering upon the descriptions of the special kinds of
Jelly-fishes, we would remind our readers that the radiate plan of
structure is reproduced in this class of animals as distinctly as in
the Polyps, though under a different aspect. Here also we find that
there is a central digestive cavity from which all the radiating
cavities, whether simple or ramified, diverge toward the periphery. It
is true that the open chambers of the Polyps are here transformed into
narrow tubes, by the thickening of the dividing partitions; or in
other words, the open spaces of the Polyps correspond to tubes in the
Acalephs, while the partitions in the Polyps correspond to the thick
masses of the body dividing the tubes in the Acalephs. But the
principle of radiation on which the whole branch of Radiates is
constructed controls the organization of Acalephs no less than that of
the other classes, so that a transverse section across any Polyp (Fig.
1), or across any Acaleph (Fig. 50), or across any Echinoderm (Fig.
140), shows their internal structure to be based upon a radiation of
all parts from the centre to the periphery.
That there may be no vagueness as to the terms used hereafter, we
would add one word respecting the nomenclature of this class, whose
aliases might baffle the sagacity of a police detective. The names
Acalephs, Medusæ, or the more common appellation of Jelly-fishes,
cover the same ground, and are applied indiscriminately to the animals
they represent. The name Jelly-fish is an inappropriate one, though
the gelatinous consistency of these animals is accurately enough
expressed by it; but they have no more structural relation to a fish
than to a bird or an insect. They have, however, received this name
before the structure of animals was understood, when all animals
inhabiting the waters were indiscriminately called fishes, and it is
now in such general use that it would be difficult to change it. The
name Medusa is derived from their long tentacular appendages,
sometimes wound up in a close coil, sometimes thrown out to a great
distance, sometimes but half unfolded, and aptly enough compared to
the snaky locks of Medusa. Their third and oldest appellation, that of
Acalephs,--alluding to their stinging or nettling property, and given
to them and like animals by Aristotle, in the first instance, but
afterwards applied by Cuvier in a more limited sense to
Jelly-fishes,--is the most generally accepted, and perhaps the most
appropriate of all.
The subject of nomenclature is not altogether so dry and arid as it
seems to many who do not fully understand the significance of
scientific names. Not only do they often express with terse precision
the character of the animal or plant they signify, but there is also
no little sentiment concealed under these jaw-breaking appellations.
As seafaring men call their vessels after friends or sweethearts, or
commemorate in this way some impressive event, or some object of their
reverence, so have naturalists, under their fabrication of appropriate
names, veiled many a graceful allusion, either to the great leaders of
our science, or to some more intimate personal affection. The _Linnæa
borealis_ was well named after his famous master, by a disciple of the
great Norwegian naturalist; _Goethea semperflorens_, the
ever-blooming, is another tribute of the same kind, while the pretty,
graceful little Lizzia, named by Forbes, is one instance among many of
a more affectionate reference to nearer friends. The allusions of this
kind are not always of so amiable a character, however,--witness the
"Buffonia," a low, noxious weed, growing in marshy places, and named
by Linnæus after Buffon, whom he bitterly hated. Indeed, there is a
world of meaning hidden under our zoölogical and botanical
nomenclature, known only to those who are intimately acquainted with
the annals of scientific life in its social as well as its
professional aspect.
* * * * *
CTENOPHORÆ.
The Ctenophoræ differ from other Jelly-fishes in their mode of
locomotion. All the Discophorous Medusæ, as well as Hydroids, move by
a rhythmical rise and fall of the disk, contracting and expanding with
alternations so regular, that it reminds one of the action of the
lungs, and seems at first sight to be a kind of respiration in which
water takes the place of air. The Greeks recognized this peculiar
character in their name, for they called them Sea-lungs. Indeed,
locomotion, respiration, and circulation are so intimately connected
in all these lower animals, that whatever promotes one of these
functions affects the other also, and though the immediate result of
the contraction and expansion of the disk seems to be to impel them
through the water, yet it is also connected with the introduction of
water into the body, which there becomes assimilated with the food in
the process of digestion, and is circulated throughout all its parts
by means of ramifying tubes. In the Ctenophoræ there is no such
regular expansion and contraction of the disk; they are at once
distinguished from the Discophoræ by the presence of external
locomotive appendages of a very peculiar character. They move by the
rapid flapping of countless little oars or paddles, arranged in
vertical rows along the surface of the disk, acting independently of
each other; one row, or even one paddle, moving singly, or all of them
together, at the will of the animal; thus enabling it to accelerate or
slacken its movements, to dart through the water rapidly, or to
diminish its speed by partly furling its little sails, or, spreading
them slightly, to poise itself with a faint, quivering movement that
reminds one of the pause of the humming-bird in the air,--something
that is neither positive motion, nor actual rest.[3]
[Footnote 3: The flappers of one side are sometimes in full
activity, while those of the other side are perfectly quiet or
nearly so, thus producing rotatory movements in every direction.]
These locomotive appendages are intimately connected with the
circulating tubes, as we shall see when we examine the structural
details of these animals, so that in them also breathing and moving
are in direct relation to each other. To those unaccustomed to the
comparison of functions in animals, the use of the word breathing, as
applied to the introduction of water into the body, may seem
inappropriate, but it is by the absorption of aerated water that these
lower animals receive that amount of oxygen into the system, as
necessary to the maintenance of life in them, as a greater supply is
to the higher animals. The name of Ctenophoræ or comb-bearers, is
derived from these rows of tiny paddles which have been called combs
by some naturalists, because they are set upon horizontal bands of
muscles, see Fig. 29, reminding one of the base of a comb, while the
fringes are compared to its teeth. These flappers add greatly to the
beauty of these animals, for a variety of brilliant hues is produced
along each row by the decomposition of the rays of light upon them
when in motion. They give off all the prismatic colors, and as the
combs are exceedingly small, so that at first sight one hardly
distinguishes them from the disk itself, the exquisite play of color,
rippling in regular lines over the surface of the animal, seems at
first to have no external cause.
_Pleurobrachia_. (_Pleurobrachia rhododactyla Ag_.)
[Illustration: Fig. 27. Pleurobrachia seen at right angles to the
plane in which the tentacles are placed. (_Agassiz_.)]
Among the most graceful and attractive of these animals are the
Pleurobrachia (Fig. 29), and, though not first in order, we will give
it the precedence in our description, because it will serve to
illustrate some features of the other two groups. The body of the
Pleurobrachia consists of a transparent sphere, varying, however, from
the perfect sphere in being somewhat oblong, and also by a slight
compression on two opposite sides (Figs. 27 and 28), so as to render
its horizontal diameter longer in one direction than in the other
(Fig. 30). This divergence from the globular form, so slight in
Pleurobrachia as to be hardly perceptible to the casual observer,
establishing two diameters of different lengths at right angles with
each other, is equally true of the other genera. It is interesting and
important, as showing the tendency in this highest group of Acalephs
to assume a bilateral character. This bilaterality becomes still more
marked in the highest class of Radiates, the Echinoderms. Such
structural tendencies in the lower animals, hinting at laws to be more
fully developed in the higher forms, are always significant, as
showing the intimate relation between all parts of the plan of
creation. This inequality of the diameters is connected with the
disposition of parts in the whole structure, the locomotive fringes
and the vertical tubes connected with them being arranged in sets of
four on either side of a plane passing through the longer diameter,
showing thus a tendency toward the establishment of a right and left
side of the body, instead of the perfectly equal disposition of parts
around a common centre, as in the lower Radiates.
[Illustration: Fig. 28. Pleurobrachia seen in plane of tentacles.
(_Agassiz_.)]
The Pleurobrachia are so transparent, that, with some preparatory
explanation of their structure, the most unscientific observer may
trace the relation of parts in them. At one end of the sphere is the
transverse split (Fig. 27), that serves them as a mouth; at the
opposite pole is a small circumscribed area, in the centre of which is
a dark eye-speck. The eight rows of locomotive fringes run from pole
to pole, dividing the whole surface of the body like the ribs on a
melon. (Figs. 27, 28.) Hanging from either side of the body, a little
above the area in which the eye-speck is placed, are two most
extraordinary appendages in the shape of long tentacles, possessing
such wonderful power of extension and contraction that, while at one
moment they may be knotted into a little compact mass no bigger than a
pin's head, drawn up close against the side of the body, or hidden
within it, the next instant they may be floating behind it in various
positions to a distance of half a yard and more, putting out at the
same time soft plumy fringes (Fig. 29) along one side, like the beard
of a feather. One who has never seen these animals may well be
pardoned for doubting even the most literal and matter-of-fact account
of these singular tentacles. There is no variety of curve or spiral
that does not seem to be represented in their evolutions. Sometimes
they unfold gradually, creeping out softly and slowly from a state of
contraction, or again the little ball, hardly perceptible against the
side of the body, drops suddenly to the bottom of the tank in which
the animal floating, and one thinks for a moment, so slight is the
thread-like attachment, that it has actually fallen from the body; but
watch a little longer, and all the filaments spread out along the side
of the thread, it expands to its full length and breadth, and resumes
all its graceful evolutions.
[Illustration: Fig. 29. Natural attitude of Pleurobrachia when in
motion.]
[Illustration: Fig. 30. Pleurobrachia seen from the extremity
opposite the mouth.]
One word of the internal structure of these animals, to explain its
relation to the external appendages. The mouth opens into a wide
digestive cavity (Figs. 27, 28), enclosed between two vertical tubes.
Toward the opposite end of the body these tubes terminate or unite in
a single funnel-like canal, which is a reservoir as it were for the
circulating fluid poured into it through an opening in the bottom of
the digestive cavity. The food in the digestive cavity becomes
liquefied by mingling with the water entering with it at the mouth,
and, thus prepared, it passes into this canal, from which, as we shall
presently see, all the circulating tubes ramifying throughout the body
are fed. Two of these circulating tubes, or, as they are called from
the nature of the liquid they contain, chymiferous tubes, are very
large, starting horizontally and at right angles with the digestive
cavity from the point of junction between the vertical tubes (Fig. 30)
and the canal. Presently they give off two branches, those again
ramifying in two directions as they approach the periphery, so that
each one of the first main tubes has multiplied to four, before its
ramifications reach the surface, thus making in all eight radiating
tubes. So far, these eight tubes are horizontal, all diverging on the
same level; but as they reach the periphery each one gives rise to a
vertical tube, running along the surface of the body from pole to
pole, just within the rows of locomotive fringes on the outer surface,
and immediately connected with them (Figs. 27, 28). As in all the
Ctenophoræ, these fringes keep up a constant play of color by their
rapid vibrations. In Pleurobrachia the prevailing tint is a yellowish
pink, though it varies to green, red, and purple, with the changing
motions of the animal. We have seen that the vertical tubes between
which the digestive cavity is enclosed, start like the cavity itself
from that pole of the body where the mouth is placed, and that, as
they approach the opposite pole, at a distance from the mouth of about
two thirds the whole length of the body, they unite in the canal,
which then extends to the other pole where the eye-speck is placed. As
it is just at this point of juncture between the tubes and the canal
that the two main horizontal tubes arise from which all the others
branch on the same plane (Figs. 27, 28), it follows that they reach
the periphery, not on a level with the pole opposite the mouth, but
removed from it by about one third the height of the body. In
consequence of this the eight vertical tubes arising from the
horizontal ones, in order to run the entire length of the body from
pole to pole, extend in opposite directions, sending a branch to each
pole, though the branch running toward the mouth is of course the
longer of the two. The tentacles have their roots in two sacs within
the body, placed at right angles with the split of the mouth. (Figs.
27, 30.) They open at the surface on the opposite side from the mouth,
though not immediately within the area at which the eye-speck is
placed, but somewhat above it, and at a little distance on either side
of it. The tentacles may be drawn completely within these sacs, or be
extended outside, as we have seen, to a greater or less degree, and in
every variety of curve or spiral.
_Bolina_. (_Bolina alata_ AG.)
[Illustration: Fig. 31. Bolina seen from the broad side; _o_
eye-speck, _m_ mouth, _r_ auricles, _v_ digestive cavity, _g_ _h_
short rows of flappers, _a_ _f_ long rows of flappers, _n_ _x_ _t_
_z_ tubes winding in the larger lobes; about half natural size.
(_Agassiz_.)]
[Illustration: Fig. 32. Bolina seen from the narrow side; _c_ _h_
short rows of flappers, _a_ _b_ long rows of flappers; other
letters as in Fig. 31. (_Agassiz_.)]
The Bolina (Fig 32), like the Pleurobrachia, is slightly oval in form,
with a longitudinal split at one end of the body, forming a mouth
which opens into a capacious sac or digestive cavity. But it differs
from the Pleurobrachia in having the oral end of the body split into
two larger lobes (Fig. 31), hanging down from the mouth. These lobes
may gape widely, or they may close completely over the mouth so as to
hide it from view, and their different aspects under various degrees
of expansion or contraction account for the discrepancies in the
description of these animals. We have seen that the Pleurobrachia
moves with the mouth upward; but the Bolina, on the contrary, usually
carries the mouth downward, though it occasionally reverses its
position, and in this attitude, with the lobes spread open, it is
exceedingly graceful in form, and looks like a white flower with the
crown fully expanded. These broad lobes are balanced on the other
sides of the body by four smaller appendages, divided in pairs, two on
each side (Fig. 32), called auricles. These so-called auricles are in
fact organs of the same kind as the larger lobes, though less
developed. The rows of locomotive flappers on the Bolina differ in
length from each other (Fig. 31), instead of being equal, as in the
Pleurobrachia. The four longest ones are opposite each other on those
sides of the body where the larger lobes are developed, the four short
ones being in pairs on the sides where the auricles are placed. At
first sight they all seem to terminate at the margin of the body, but
a closer examination shows that the circulating tubes connected with
the longer row extend into the lobes, where they wind about in a
variety of complicated involutions. (Fig. 32.) The movements of the
Bolina are more sluggish than those of the Pleurobrachia, and the long
tentacles, so graceful an ornament to the latter, are wanting in the
former. With these exceptions the description given above of the
Pleurobrachia will serve equally well for the Bolina. The structure is
the same in all essential points, though it differs in the size and
proportion of certain external features, and its play of color is less
brilliant than that of the Pleurobrachia. The Bolina, with its slow,
undulating motion, its broad lobes sometimes spreading widely, at
other times folded over the mouth, its delicacy of tint and texture,
and its rows of vibrating fringes along the surface, is nevertheless a
very beautiful object, and well rewards the extreme care without which
it dies at once in confinement.
_Idyia_. (_Idyia roseola_ AG.)
[Illustration: Fig. 33. Idyia roseola seen from the broad side,
half natural size; _a_ anal opening, _b_ lateral tube, _c_
circular tube, _d_ _e_ _f_ _g_ _h_ rows of locomotive flappers.
(_Agassiz_.)]
The lowest genus of Ctenophoræ found on our coast, the Idyia (Fig.
33), has neither the tentacles of the Pleurobrachia, nor the lobes of
the Bolina. It is a simple ovate sphere, the interior of which is
almost entirely occupied by an immense digestive cavity. It would seem
that the reception and digestion of food is intended to be the almost
exclusive function of this animal, for it has a mouth whose ample
dimensions correspond with its capacious stomach. Instead of the
longitudinal split serving as a mouth, in the Bolina and
Pleurobrachia, one end of the body in the Idyia is completely open
(Fig. 33), so that occasionally some unsuspicious victim of smaller
diameter than itself may be seen to swim into this wide portal, when
suddenly the door closes behind him with a quick contraction, and he
finds himself a prisoner. The Idyia does not always obtain its food
after this indolent fashion however, for it often attacks a Bolina or
Pleurobrachia as large or even larger than itself, when it extends its
mouth to the utmost, slowly overlapping the prey it is trying to
swallow by frequent and repeated contractions, and even cutting off by
the same process such portions as cannot be forced into the digestive
cavity.
The general internal structure of the Idyia corresponds with that of
the Bolina and Pleurobrachia; it has the same tubes branching
horizontally from the main cavity, then ramifying as they approach the
periphery till they are multiplied to eight in all, each of which
gives off one of the vertical tubes connected with the eight rows of
locomotive flappers. Opposite the mouth is the eye-speck, placed as in
the two other genera, at the centre of a small circumscribed area,
which in the Idyia is surrounded by delicate fringes, forming a
rosette at this end of the body. These animals are exceedingly
brilliant in color; bright pink is their prevailing hue, though pink,
red, yellow, orange, green, and purple, sometimes chase each other in
quick succession along their locomotive fringes. At certain seasons,
when most numerous, they even give a rosy tinge to patches on the
surface of the sea. Their color is brightest and deepest before the
spawning season, but as this advances, and the ovaries and spermaries
are emptied, they grow paler, retaining at last only a faint pink
tint. They appear early in July, rapidly attain their maximum size,
and are most numerous during the first half of August. Toward the end
of August they spawn, and the adults are usually destroyed by the
early September storms, the young disappearing at the same time, not
to be seen again till the next summer. It is an interesting question,
not yet solved, to know what becomes of the summer's brood in the
following winter. They probably sink into deep waters during this
intervening period. The Idyia, like the Pleurobrachia, moves with the
mouth upward, but inclined slightly forward also, so as to give an
oblique direction to the axis of the body.[4]
[Footnote 4: Until this summer only the three genera of Ctenophoræ
above mentioned were supposed to exist along our coast, but during
the present season I have had the good fortune to find two
additional ones. One of them, the Lesueuria, resembles a Bolina
with the long lobes so cut off, that they have a very stunted
appearance in comparison with those of the Bolina. The other, the
Mertensia, is closely allied to Pleurobrachia; it is exceedingly
flattened and pear-shaped. This species was discovered long ago by
Fabricius, but had escaped thus far the attention of other
naturalists. (_A. Agassiz_.)]
EMBRYOLOGY OF CTENOPHORÆ.
All the Ctenophoræ are reproduced from eggs, these eggs being so
transparent that one may follow with comparative ease the changes
undergone by the young while still within the egg envelope.
Unfortunately, however, they are so delicate that it is impossible to
keep them alive for any length of time, even by supplying them
constantly with fresh sea-water, and keeping them continually in
motion, both of which are essential conditions to their existence. It
is therefore only from eggs accidentally fished up at different stages
of growth that we may hope to ascertain any facts respecting the
sequence of their development. When hatched, the little Ctenophore is
already quite advanced. It is small when compared with the size of the
egg envelope, and long before it is set free, it swims about with
great velocity within the walls of its diminutive prison (Fig. 35).
The importance of studying the young stages of animals can hardly find
a better illustration than among the Ctenophoræ. Before their
extraordinary embryonic changes were understood, many of the younger
forms had found their way into our scientific annals as distinct
animals, and our nomenclature thus became burdened with long lists of
names which will disappear as our knowledge advances.
The great size of their locomotive flappers in proportion to the rest
of the body, is characteristic of the young Ctenophoræ. They seem like
large paddles on the sides of these tiny transparent spheres, and,
owing to their great power as compared with those of the adult, the
young move with extraordinary rapidity. The Pleurobrachia alone
retains its quickness of motion in after life, and although its long
graceful streamers appear only as short stumpy tentacles in the young
(Fig. 34), yet its active little body would be more easily recognized
in the earlier stages of growth than that of the other Ctenophoræ.
Figs. 34, 35, and 36 show the Pleurobrachia at various stages of
growth; Fig. 34, with its thick stunted tentacles and short rows of
flappers, is the youngest; the flappers themselves are rather long at
this age, looking more like stiff hairs than like the minute fringes
of the adult. In Fig. 35 the tentacles are already considerably longer
and more delicate; in Fig. 36 the vertical tubes are already
completed, while Figs. 27-29 present it in its adult condition.
[Illustration: Fig. 34. Young Pleurobrachia still in the egg; _t_
tentacle, _e_ eye-speck, _c_ _c_ rows of locomotive flappers, _d_
digestive cavity; greatly magnified.]
[Illustration: Fig. 35. Young Pleurobrachia swimming about in the
egg just before hatching; magnified.]
[Illustration: Fig. 36. Young Pleurobrachia resembling somewhat
the adult; _f_ funnel leading to anal opening, _l_ lateral tubes,
_o_ _o_ _o'_ _o'_ rows of locomotive flappers; magnified.]
[Illustration: Fig. 37. Young Idyia, greatly magnified; lettering
as in Fig. 36; _d_ digestive cavity.]
The Idyia differs greatly in appearance at different periods of its
development, and, indeed, no one would suspect, without some previous
knowledge of its transformations, that the young Idyia, with its rapid
gyrations, its short ambulacral tubes, like immense pouches (Fig. 37),
its large pigment spots scattered over the surface (Fig. 38), was an
earlier stage of the rosy-hued Idyia, which glides through the water
with a scarcely perceptible motion. Figs. 37-40 represent the various
stages of its growth. It will be seen how very short are the
locomotive fringes (Fig. 39) in comparison with those of the
full-grown ones (Fig. 33). It is only in the adult Idyia that these
rows attain their full height, and the tubes, ramifying throughout the
body (Fig. 40), are completed.
[Illustration: Fig. 38. Young Idyia seen from the anal extremity,
magnified; _a_ anal opening, other letters as in Fig. 36.]
[Illustration: Fig. 39. Idyia somewhat older than Fig. 37,
lettering as before; magnified.]
[Illustration: Fig. 40. Young Idyia in which the ambulacral tubes
begin to ramify; magnified, letters as before.]
The Bolina, in its early condition, recalls the young Pleurobrachia.
At this period it has the same rapid motion, and when somewhat more
advanced, long tentacles, resembling those of the Pleurobrachia, make
their appearance (Fig. 41); it is only at a later period that the
tentacles become contracted, while the large lobes (Fig. 42), so
characteristic of Bolina, are formed by the elongation of the oral end
of the body, the auricles becoming more conspicuous at the same time
(Fig. 43). A little later the lobes enlarge, the movements become more
lazy; it assumes both in form and habits the character of the adult
Bolina.
[Illustration: Fig. 41. Young Bolina in stage resembling
Pleurobrachia; greatly magnified.]
[Illustration: Fig. 42. Young Bolina seen from the broad side,
with rudimentary auricles and lobes; magnified.]
[Illustration: Fig. 43. The same as Fig. 42, seen from the narrow
side.]
The series of changes through which the Ctenophoræ pass are as
remarkable as any we shall have occasion to describe, though not
accompanied with such absolutely different conditions of existence.
The comparison of the earlier stages of life in these animals with
their adult condition is important, not only with reference to their
mode of development, but also because it gives us some insight into
the relative standing of the different groups, since it shows us that
certain features, permanent in the lower groups, are transient in the
higher ones. A striking instance of this occurs in the fact mentioned
above, that though the long tentacles so characteristic of the adult
Pleurobrachia exist in the young Bolina, they yield in importance at a
later period to the lobes which eventually become the predominant and
characteristic feature of the latter.
* * * * *
DISCOPHORÆ.
The disk of the Discophoræ is by no means so delicate as that of the
other Jelly-fishes. It seems indeed quite solid, and somewhat like
cartilage to the touch, and yet so large a part of its bulk consists
of water, that a Cyanea, weighing when alive about thirty-four pounds,
being left to dry in the sun for some days, was found to have lost
about 99/100 of its original weight,--only the merest film remaining
on the paper upon which it had been laid. The prominence of the disk
in this group of Jelly-fishes is well characterized by their German
name, "Scheiben quallen," viz. disk-medusæ. We shall see hereafter
that the disk, so large and seemingly solid in the Discophoræ, thins
out in many of the other Jelly-fishes, and becomes exceedingly
concave. This is especially the case in many of the Hydroid Medusæ,
where it assumes a bell-shaped form, and is constantly spoken of as
the bell. It should be remembered, however, in reading descriptions of
these animals, that the so-called bell is only a modified disk, and
perfectly homologous with that organ in the Discophoræ.
The Discophorous Medusæ are distinguished from all others by the
peculiar nature of the reproductive organs. They are contained in
pouches (Fig. 50, _o_, _o_, _o_, _o_), the contents of which are first
discharged into the main cavity, and then pass out through the mouth.
Pillars support the four angles of the digestive cavity, thus
separating the lower from the upper floor of the disk, while the
chymiferous tubes (Fig. 50) branch and run into each other near the
periphery, forming a more or less complicated anastomosing network,
instead of a simple circular tube, as is the case with the Hydroid
Medusæ. (Fig. 74.)
_Cyanea_. (_Cyanea arctica_ PÉR. et LES.)
In our descriptions of the Discophoræ, we may give the precedence to
the Cyanea on account of its size. This giant among Jelly-fishes is
represented in Fig. 44. It is much to be regretted that these animals,
when they are not so small as to escape attention altogether, are
usually seen out of their native element, thrown dead or dying on the
shore, a mass of decaying gelatinous matter. All persons who have
lived near the sea are familiar with the so-called Sea-blubbers,
sometimes strewing the sandy beaches after the autumn storms in such
numbers that it is difficult to avoid them in walking or driving. In
such a condition the Cyanea is far from being an attractive object; to
form an idea of his true appearance, one must meet him as he swims
along at midday, rather lazily withal, his huge semi-transparent disk,
with its flexible lobed margin, glittering in the sun, and his
tentacles floating to a distance of many yards behind him.
Encountering one of those huge Jelly-fishes, when out in a row-boat
one day, we attempted to make a rough measurement of his dimensions
upon the spot. He was lying quietly near the surface, and did not seem
in the least disturbed by the proceeding, but allowed the oar, eight
feet in length, to be laid across the disk, which proved to be about
seven feet in diameter. Backing the boat slowly along the line of the
tentacles, which were floating at their utmost extension behind him,
we then measured these in the same manner, and found them to be rather
more than fourteen times the length of the oar, thus covering a space
of some hundred and twelve feet. This sounds so marvellous that it may
be taken as an exaggeration; but though such an estimate could not of
course be absolutely accurate, yet the facts are rather understated
than overstated in the dimensions here given. And, indeed, the
observation was more careful and precise than the circumstances would
lead one to suppose, for the creature lay as quietly, while his
measure was taken, as if he had intended to give every facility for
the operation. This specimen was, however, of unusual size; they more
commonly measure from three to five feet across the disk, while the
tentacles may be thirty or forty feet long. The tentacles are
exceedingly numerous (see Fig. 44), arising in eight distinct bunches,
from the margin of the disk, and hanging down in a complete labyrinth.
These animals are not so harmless as it would seem, from their soft,
gelatinous consistency; it is no pleasant thing when swimming or
bathing to become entangled in this forest of fine feelers, for they
have a stinging property like nettles, and may render a person almost
insensible, partly from pain, and partly from a numbness produced by
their contact, before he is able to free himself from the network in
which he is caught. The weapons by which they produce these results
seem so insignificant, that one cannot but wonder at their power. The
tentacles are covered by minute cells, lasso-cells as they are called,
(similar to those of Astrangia, Fig. 19,) each one of which contains a
whip finer than the finest thread, coiled in a spiral within it.
[Illustration: Fig. 44. Cyanea arctica; greatly reduced in size.]
These myriad whips can be thrown out at the will of the animal, and
really form an efficient galvanic battery. Behind the veil of
tentacles, and partly hidden by it, four curtains, with lobed or
ruffled margins, dimly seen in Fig. 44, hang down from the under
surface of the disk. The ovaries are formed by four pendent pouches,
placed near the sides of the mouth, and attached to four cavities
within the disk. Around the circumference of the disk are eight
eye-specks, each formed by a small tube protected under a little
lappet or hood rising from the upper surface of the disk. The
prevailing color of this huge Jelly-fish is a dark brownish-red, with
a light, milk-white margin, tinged with blue, the tentacles and other
pendent appendages having a somewhat different hue from the disk. The
ovaries are flesh-colored, the curtain formed by the expansion of the
lobes of the mouth is dark brown, while the tentacles are of different
colors, some being yellow, others purple, and others reddish brown or
pink.
[Illustration: Fig. 45. Scyphistoma of a Discophore; Aurelia
flavidula. (_Agassiz_.)]
[Illustration: Fig. 47. Strobila of a Discophore; Aurelia
flavidula. (_Agassiz_.)]
[Illustration: Fig. 46. Scyphistoma, older than Fig. 45.
(_Agassiz_.)]
Strange to say, this gigantic Discophore is produced by a Hydroid
measuring not more than half an inch in height when full grown; could
we follow the history of any egg laid by one of these Discophoræ in
the autumn, and this has indeed been partially done, we should see
that, like any other planula, the young hatched from the egg is at
first spherical, but presently becomes pear-shaped, and attaches
itself to the ground. From the upper end tentacles project (see Fig.
45), growing more numerous, as in Fig. 46, though they never exceed
sixteen in number. As it increases in height constrictions take place
at different distances along its length, every such constriction being
lobed around its margin, till at last it looks like a pile of
scalloped saucers or disks strung together (see Fig. 47). The topmost
of these disks falls off and dies; but all the others separate by the
deepening of the constrictions, and swim off as little free disks
(Fig. 48), which eventually grow into the enormous Jelly-fish
described above. These three phases of growth, before the relation
between them was understood, have been mistaken for distinct animals,
and described as such under the names of Scyphistoma, Strobila, and
Ephyra.
[Illustration: Fig. 48. Ephyra of a Discophore; Aurelia flavidula.
(_Agassiz_.)]
_Aurelia_. (_Aurelia flavidula_ PÉR. et LES.)
[Illustration: Fig. 49. Aurelia seen in profile, reduced.
(_Agassiz_.)]
Another large Discophore, though by no means to be compared to the
Cyanea in size, is our common Aurelia (Figs. 49, 50) Its bluish-white
disk measures from twelve to fifteen inches in diameter, but its
dimensions are not increased by the tentacles, which have no great
power of contraction and expansion, and form a short fringe around its
margin, widening and narrowing slightly as the tentacles are stretched
or drawn in. It is quite transparent, as may be seen in Fig 49, where
all the fine ramifications of the chymiferous tubes as well as the
ovaries, are seen through the vault of the disk. Fig. 50 represents
the upper surface, with the ovaries around the mouth, occupying the
same position as those of the Cyanea, though they differ from the
latter in their greater rigidity, and do not hang down in the form of
pouches. The males and females in this kind of Jelly-fish may be
distinguished by the difference of color in the reproductive organs,
which are rose-colored in the males, and of a dull yellow in the
females. The process of development is exactly the same in the Aurelia
as in the Cyanea, though there is a very slight difference in their
respective Hydroids. They are, however, so much alike, that one is
here made to serve for both, the above figures being taken from the
Hydroid of the Aurelia. It is curious, that while, as in the case of
the Aurelia and Cyanea, very dissimilar Jelly-fishes may arise from
almost identical Hydroids, we have the reverse of the proposition, in
the fact that Hydroids of an entirely distinct character may produce
similar Jelly-fishes.
[Illustration: Fig. 50. Aurelia flavidula, seen from above; _o_
mouth, _e e e e_ eyes, _m m m m_ lobes of the mouth, _o o o o_
ovaries, _t t t t_ tentacles, _w w_ ramified tubes. (_Agassiz_.)]
The embryos or little planulæ, hatched from the Cyanea and Aurelia in
the fall, seem to be gregarious in their mode of life, swimming about
together in great numbers till they find a suitable point of
attachment, and assume their fixed Hydroid existence. The Cyaneæ,
however, when adult, are usually found singly, while the Aureliæ, on
the contrary, seek each other, and commonly herd together.
_The Campanella_. (_Campanella pachyderma_ A. AG.)
[Illustration: Fig. 51. Campanella seen in profile; greatly
magnified.]
[Illustration: Fig. 52. Same, seen from below.]
The Campanella (Fig. 51) is a pretty little Jelly-fish, not larger
than a pin's head, reproduced directly from eggs, without passing
through the Hydroid stage. During its early stages of growth it
probably remains attached to floating animals, thus leading a kind of
parasitic existence; but as its habits are not accurately known, this
cannot be asserted as a constant fact respecting them. The veil in
this Jelly-fish is very large, forming pendent pouches hanging from
the circular canal (see Fig. 51), and leaving but just room enough for
the passage of the proboscis between the folds. It may not be amiss to
introduce here a general account of this organ, which occurs in many
of the Medusæ, though it has very different proportions in the various
kinds. It is a delicate membrane, hanging from the circular tube, so
as partially to close the mouth of the bell, leaving a larger or
smaller opening for the passage of the water, which is taken in and
forced out again by the alternate expansions and contractions of the
bell.
There are but four chymiferous tubes in the Campanella, and four stiff
tentacles, which in consequence of the peculiar character of the veil
appear, when the animal is seen in profile, to start from the middle
of the disk. The ovaries consist of eight pouches, placed near the
point of junction of the four chymiferous tubes. (Fig. 52.) This
little Medusa is of a dark yellowish color with brownish ocellated
spots, scattered profusely over the upper part of the disk.
_Circe_. (_Trachynema digitale_ A. AG.)
[Illustration: Fig. 53. Trachynema digitale; about twice the
natural size.]
Among the Jelly-fishes, the position of which is somewhat doubtful, is
the Circe (Fig. 53), differing greatly in outline from the ordinary
Jelly-fishes. As may be seen in Figure 53, the bell forms but a small
portion of the animal; it rises in a sharp cone on the summit,
thinning out at the lower edge, to form the large cavity in which
hangs the long proboscis and the eight ovaries, four of which may be
seen in Fig. 53 crowded with eggs. The Circe differs in consistency,
as well as in form, from other Jelly-fishes. It is hard and horny to
the touch, and the veil, usually so light and filmy, is here a thick
folded membrane, which at every stroke of the animal forces the water
in and out of the cavity. It is very active, moving by powerful jerks,
each one of which throws it far on its way. It advances usually in
straight lines; or, if it wishes to change its direction, it drives
the water out of the veil suddenly on one side or the other, so as to
shoot off, sometimes at right angles with its former path. Four large
pedunculated eyes, hidden in the figure by the tentacles, stand out
prominently from the circular tube. When the animal is in motion, the
tentacles are carried closely curled around the edge of the disk, as
in Fig. 53, where the Circe is represented under a magnifying power of
two and a half diameters. This Jelly-fish is of a delicate rose color,
the tentacles assuming, however, a dark-purple tint at their
extremities when contracted.
_Lucernaria_. (_Haliclystus auricula_ CLARK.)
[Illustration: Fig. 54. Group of Lucernaria attached to eel-grass;
natural size.]
[Illustration: Fig. 55. Lucernaria seen from the mouth side.]
One of the prettiest and most graceful, as well as one of the most
common of our Jelly-fishes, is the Lucernaria (Fig. 54). It has such
an extraordinary contractility of all its parts, that it is not easy
to describe it under any definite form or position, since both are
constantly changing; but perhaps of all its various attitudes and
outlines none are more normal to it than those given in Fig. 54. It
frequently raises itself in the upright position represented here by
the individual highest on the stem, spreading itself in the form of a
perfectly symmetrical cup or vase, the margin of which is indented by
a succession of inverted scallops, the point of junction between every
two scallops being crowned by a tuft of tentacles. But watch it for a
while, and the sides of this vase turn backward, spreading completely
open, till they present the whole inner surface, with the edges even
curved a little downward, drooping slightly, and the proboscis rising
in the centre. In such an attitude one may trace with ease the shape
of the mouth, the lobes surrounding it, as well as the tubes and
cavities radiating from it toward the margin. A touch is, however,
sufficient to make it close upon itself, shrinking together in the
attitude of the third individual in Fig. 54, or even drawing its
tentacles completely in, and contracting all its parts till it looks
like a little ball hanging on the stem. These are but a few of its
manifold changes, for it may be seen in every phase of expansion and
contraction. Let us now look for a moment at the details of its
structure. The resemblance to a cup or vase, as in the upper figure of
the wood-cut (Fig. 54), is deceptive; for a vase is hollow, whereas
the Lucernaria, though so delicate and transparent that its upper
surface, when thus stretched, seems like a mere film, is nevertheless
a solid gelatinous mass, traversed by certain channels, cavities, and
partitions, but otherwise continuous throughout. The peduncle by which
it is attached is but an extension of the floor of a gelatinous disk,
corresponding to that of any Jelly-fish. Four tubes pass through the
whole length of this peduncle, and open into four chambers, dividing
the digestive cavity above into as many equal spaces. (Fig. 55.) These
spaces are produced by folds in the upper floor of the disk, uniting
it to the lower floor at given distances, and forming so many
partition-walls, dividing the digestive sac into four distinct
cavities. These lines of juncture between the two floors, where the
partitions occur, produce the four radiating lines, running from the
proboscis to the margin of the disk, on the upper surface. (Fig. 55.)
The triangular figures, running from the mouth to each cluster of
tentacles, are produced by the ovaries, which consist of distinct
pouches or bags attached to the upper surface of the disk, and hanging
down into the cavities below; every little dot within these triangular
spaces represents such a bag. Each bag is crowded with eggs, which
drop into the digestive cavity at the spawning season, and are passed
out at the mouth. The tentacles always grow in clusters, but are
nevertheless arranged according to a regular order. They are
club-shaped at their extremities, but are hollow throughout, opening
into the chambers of the digestive cavity, two of the clusters thus
being connected with each chamber. Their chief office seems to be to
catch the food and convey it to the mouth, though they may also be
used as a kind of suckers, and the animal not unfrequently attaches
itself by means of these appendages. Between every two clusters of
tentacles will be observed a short, single appendage, of an entirely
different appearance. These are the so-called auricles, and though so
unlike tentacles in the adult animal, when in their earlier stages
(Fig. 56) they resemble each other closely. But as their development
goes on, the tentacles stretch out into longer, more delicate flexible
organs, while the auricles remain short and compact throughout life.
They contain a slight pigment spot representing an eye, though how far
it serves the purpose of vision remains doubtful. They are chiefly
used by the animal as a means of adhering to any surface upon which it
may wish to fasten itself; for the Lucernaria, though usually found
attached to eel-grass in shoal water, has the power of independent
motion, and frequently separates from its resting-place, floating
about freely in the water for a while, or attaching itself anew by
means of the auricles and tentacles upon some other object. The color
of this pretty Acaleph varies from a greenish hue to green, with a
faint tinge of red, or to a reddish brown. One of its commonest and
most exquisite tints is that of a pale aqua-marine. It may be found
along our shores wherever the eel-grass grows, and as far out as this
plant extends. It thrives admirably in confinement, and for this
reason is especially adapted to the aquarium.
[Illustration: Fig. 56. Young Lucernaria; magnified.]
* * * * *
HYDROIDS.
Under this order, the general character of which has already been
explained in the introductory chapter on Acalephs, are included a
number of groups which, whether as Hydroid communities in their
earlier phases of existence, or as free swimming Medusæ in their
farther development, challenge our admiration, both for their beauty
of form and color, and their grace of motion. Some of them are so
minute that they escape the observation of all but those who are
laboriously seeking for the hidden treasures of the microscopic world,
but the greater number are large enough to be readily found by the
most inexperienced collector, when his attention is once drawn to
them; and he may easily stock his aquarium with these pretty little
communities, and even trace the development of the Jelly-fishes upon
them.
To the Hydroids belong the Campanularians, the Sertularians, and the
Tubularians. Some examples of each, as represented on our shores, will
be found under their different heads, accompanied with full
descriptions. There is another group usually considered as distinct
from Hydroids, and known as a separate order among Acalephs, under the
name of Siphonophoræ, but included with them here in accordance with
the views of Vogt, Agassiz, and others, in whose opinion they differ
from the ordinary Hydroid communities only in being free and floating,
instead of fixed to the ground. Some new facts, published here for the
first time, tend to sustain the accuracy of this classification.[5]
With these few preliminary remarks to show the connection of the
order, let us now look at some of the animals belonging to it more in
detail.
[Footnote 5: See Chapter on Nanomia.]
_Campanularians_.
All the Campanularians, of which Oceania (Fig. 68), Clytia (Fig. 73),
and Eucope (Fig. 61) form a part, belong among those little shrub-like
communities of animals called Hydroids, from which most of our
Jelly-fishes are developed. They differ in one essential feature from
the Tubularians. (Fig. 93.) The whole stem, from summit to base, is
enveloped in a horny sheath, extending around both the fertile and
sterile individuals of the community, and forming a network at the
base of the stem, which serves as a kind of foundation for the whole
stock. To the naked eye such a community looks like a tiny shrub (see
Fig. 57), with the branches growing in regular alternation on either
side of the stems. The reproductive calycles, i.e. the protecting
envelopes covering the young Medusæ, usually arise in the angles of
the branches formed by a prolongation of the sheath. These calycles or
bells, as they are called, assume a great variety of
shapes,--elliptical, round, pear-shaped, or ringed like the Clytia.
(Fig. 72.) In one such bell there may be no less than twenty or thirty
Medusæ developed one below the other; when ready to hatch, the calycle
bursts and allows them to escape.
_Eucope_. (_Eucope diaphana_ AG.)
[Illustration: Fig. 57. Hydrarium of Eucope; natural size.]
[Illustration: Fig. 58. Portion of Fig. 57; magnified.]
In Figs. 60 and 61 we have a representation of our little Eucope, one
of the prettiest of the Jelly-fishes belonging to this group; Fig. 57
represents the Hydroid from which it arises; a single branch with the
reproductive bell being magnified in Fig. 58. In Fig. 59 is seen a
portion of the Jelly-fish disk, with the fringe of tentacles highly
magnified. The disk of the Eucope (Fig. 60) looks like a shallow bell,
of which the proboscis often seems to form the handle; for the disk
has such an extraordinary thinness that it turns inside out with the
greatest ease, so that the inner surface may become at any moment the
outer one, with the proboscis projecting from it, as in Fig. 60, while
the next movement of the animal may reverse its whole position, and
the proboscis then hangs down from the inside, as in other
Jelly-fishes. (See Fig. 61.)
[Illustration: Fig. 59. Part of marginal tube and tentacles of
Eucope, greatly magnified; _e_ eye-speck, _b_ base of tentacle.,
_r_ reentering base of tentacle.]
[Illustration: Fig. 60. Young Eucope; magnified.]
[Illustration: Fig. 61. Adult Eucope seen in profile; magnified.]
[Illustration: Fig. 62. Quarter disk of Fig. 60, seen from below;
_e e_ tentacles bearing eye-speck.]
The tentacles are solid and stiff like little hairs, and two of them,
in each quarter-segment of the disk, have small concretions at the
base, which are no doubt eye-specks. (See Fig. 62.) Along the
chymiferous tubes little swellings are developed, which increase
gradually, and become either ovaries or spermaries, according to the
sex of the animal. (Fig. 63.) In the adult the genital organs hang
down, like elongated bags, from the chymiferous tubes. (Fig. 64.) The
tentacles are numerous, multiplying to about a hundred and ninety-two
in the adult, and increasing according to the numerical law to be
explained in the description of the Oceania.
[Illustration: Fig. 63. Quarter-disk of young Eucope, older than
Fig. 62, with a second set of tentacles (2) between the first set
(1).]
[Illustration: Fig. 64. Magnified quarter-disk of adult Eucope.]
This little Jelly-fish is one of the most common in our Bay. There is
not a night or day when they cannot be taken in large numbers, from
the early spring till late in the autumn; and as the breeding season
lasts during the whole of that period, they are found in all possible
stages of growth. In consequence of this, the course of their
development, and the relation between the different phases of their
existence as Hydroids, and afterwards as Acalephs, are well known,
though the successive steps of their growth have not been traced
connectedly, as in some of the other Jelly-fishes, the Tima or
Melicertum, for instance. The process is, however, so similar
throughout the class of Hydroids, that, having followed it from
beginning to end in some of the groups, we have the key to the history
of others, whose development has not been so fully traced. The eggs
laid by the Eucope in the autumn develop into planulæ, which acquire
their full size as Hydroid communities toward the close of the winter,
and the development of the young Medusæ upon them, as described above,
begins with the opening spring.
_Oceania_. (_Oceania languida_ A. AG.)
The Oceania (Fig. 68) is so delicate and unsubstantial, that with the
naked eye one perceives it only by the more prominent outlines of its
structure. We may see the outline of the disk, but not the disk
itself; we may trace the four faint thread-like lines produced by the
radiating tubes traversing the disk from the summit to the margin; and
we may perceive, with far more distinctness, the four ovaries attached
to these tubes near their base; we may see also the circular tube
uniting the radiating tubes, and the tentacles hanging from it, and we
can detect the edge of the filmy veil that fringes the margin of the
disk. But the substance connecting all these organs is not to be
distinguished from the element in which it floats, and the whole
structure looks like a slight web of threads in the water, without our
being able to discern by what means they are held together. Under the
microscope, however, the invisible presently becomes visible, and we
find that this Jelly-fish, like all others, has a solid gelatinous
disk.
[Illustration: Fig. 65. Young Oceania just escaped from its
reproductive calycle; magnified.]
[Illustration: Fig. 66. The same as Fig. 65, from below, still
more magnified; _t_ long tentacles., _t'_ rudimentary tentacle,
_e_ eye-speck on each side of base of tentacles.]
[Illustration: Fig. 67. Young Oceania, older than Fig. 65;
magnified.]
[Illustration: Diagram of succession of tentacles.]
Let us begin with its earlier condition. When it first escapes from
the parent Hydroid stock, the Oceania is almost spherical in form.
(See Fig. 65.) The disk is divided by four chymiferous tubes, running
from the summit to the margin, where they meet the circular tube in
which they all unite. At this time, it has but two well-developed
tentacles, opposite each other on the margin of the disk, just at the
base of two of the chymiferous tubes (Fig. 66), while two others are
just discernible in a rudimentary state, forming slight projections at
the base of the two other tubes. Fig. 66 gives a view of the animal
from below, at this stage of its growth, while Fig. 65 shows it in
profile. It will be seen by the latter how very spherical is the
outline of the disk at this period, while the proboscis, in which are
placed the mouth and digestive cavity, is quite long, and hangs down
considerably below the lower surface of the disk. As the animal
advances in age the disk loses its spherical outline, and becomes much
flattened, as may be seen in Fig. 67. It may be well to introduce here
some explanation of the law according to which the different sets of
tentacles follow each other in successive cycles of growth, since it
is a law of almost universal application in Jelly-fishes and Polyps;
and, owing to the smaller number and simpler arrangement of the
tentacles in Oceania, it may be more easily analyzed in them than in
many others, where the number and complication of the different sets
of tentacles make it very difficult to trace their relation to each
other during their successive growth. We have seen that the Oceania
begins life with only two tentacles. These form the first set, and are
marked with the number 1 in the subjoined diagram, which gives the
plan of all the different sets in their regular order. The second set,
marked 2, consists also of two, which are developed at equal distances
between the first two, i.e. at right angles with them. The third set,
however, marked 3, consists of four, as do all the succeeding sets,
and they are developed between the first and second. The fourth set
comes in between the first and third; the fifth between the third and
second; the sixth between the first and fourth; the seventh between
the fifth and second; the eighth between the third and fourth; the
ninth between the fifth and third. The ultimate number of tentacles in
the Oceania is thirty-two, or sometimes thirty-six, and the cycles
always in twos or multiples of two. But whatever be the number
included in the successive sets of tentacles, and the unit for the
first set ranges from two to forty-eight, the law in different kinds
of Jelly-fishes is always the same, the youngest set always forming
between the oldest preceding set. Thus the fourth set comes in between
the first and third, and the fifth between the second and third, the
intervals occupied now by the fourth set, being limited by the first
set of tentacles on one side, and by the third set on the other side,
while the intervals occupied by the fifth set are bounded by the
second and third sets.
[Illustration: Fig. 68. Adult Oceania; natural size.]
The little spheres represented between the tentacles on the margin of
the disk, in Figs. 65-67, are eye-specks, and these continue to
increase in number with age; in this the Oceania differs from the
Eucope, in which it will be remembered there were but two eye-specks
in each quarter-segment of the disk throughout life. Fig. 68
represents the adult Oceania in full size, when it averages from an
inch and a half to two inches in diameter. It is slow and languid in
its movements, coming to the surface only in the hottest hours of the
summer days; at such times it basks in the sun, turning lazily about,
and dragging its tentacles after it with seeming effort. Sometimes it
remains for hours suspended in the water, not moving even its
tentacles, and offering a striking contrast to its former great
activity when young, and to the lively little Eucope, which darts
through the water at full speed, hardly stopping to rest for a moment.
If the Oceania be disturbed it flattens its disk, and folds itself up
somewhat in the shape of a bale (see Fig. 69), remaining perfectly
still, with the tentacles stretching in every direction. When the
cause of alarm is removed, it gently expands again, resuming its
natural outline and indolent attitudes. The number of these animals is
amazing. At certain seasons, when the weather is favorable, the
surface of the sea may be covered with them, for several miles, so
thickly that their disks touch each other. Thus they remain packed
together in a dense mass, allowing themselves to be gently drifted
along by the tide till the sun loses its intensity, when they retire
to deeper waters. Some points, not yet observed, are still wanting to
complete the history of this Jelly-fish. By comparing such facts,
however, as are already collected respecting it, with our fuller
knowledge of the same process of growth in the Eucope, Tima, and
Melicertum, we may form a tolerably correct idea of its development.
It is hatched from a Campanularia.
[Illustration: Fig. 69. Attitude assumed by Oceania when
disturbed.]
_Clytia_. (_Clytia bicophora_ AG.)
In Figs. 70-73 we have the Acalephian and Hydroid stages of the Clytia
(Fig. 73), another very pretty little Jelly-fish, closely allied to
the Oceania. When first hatched, like the Oceania, it is very convex,
almost thimble-shaped (see Fig. 70), but a little later the disk
flattens and becomes more open, as in Fig. 71. In Fig. 72, we have a
branch of the Hydroid, a Campanularia, greatly magnified, with the
annulated reproductive calycle attached to it, and crowded with
Jelly-fishes ready to make their escape as soon as the calycle bursts.
The adult Clytia (Fig. 73) is somewhat smaller and more active than
the Oceania, and is easily recognized by the black base of its
tentacles, at their point of juncture with the margin of the disk. It
is more commonly found at night, than in the day-time, being nocturnal
in its habits.
[Illustration: Fig. 70. Young Clytia just escaped from the
reproductive calycle.]
[Illustration: Fig. 71. Clytia somewhat older than Fig. 70.]
[Illustration: Fig. 72. Magnified portion of Hydrarium of Clytia.]
[Illustration: Fig. 73. Adult Clytia; twice natural size.]
_Zygodactyla_. (_Zygodactyla groenlandica_ AG.)
Little has been known, and still less published, of this remarkable
genus of Jelly-fish (Figs. 74, 75) up to the present time. The name
Zygodactyla, or Twinfinger, was given to it by Brandt, from drawings
made by Mertens, who had some opportunity of studying it in his
journey around the world. These drawings were published in the
Transactions of the St. Petersburg Academy. In the year 1848 Professor
Agassiz read a paper upon one of the species of this genus belonging
to our coast, before the American Academy, in which he called it
Rhacostoma, not being aware that it had already received a name, and
gave some account of its extraordinary phosphorescent properties. The
name Rhacostoma must of course yield to that of Zygodactyla, which has
a prior claim.
[Illustration: Fig. 74. Zygodactyla seen from above.]
The average size of this Jelly-fish when full grown is from seven to
eight inches in diameter; sometimes it may measure even ten or eleven,
but this is rather rare. The light-violet colored disk is exceedingly
delicate and transparent, its edge being fringed with long fibrous
tentacles, tinged with darker violet at their point of juncture with
the disk, and hanging down a yard and more when fully extended, though
they vary in length according to the size of the specimen, and, in
consequence of their contractile power, may seem much shorter at some
moments than at others. The radiating tubes in this Jelly-fish are
exceedingly numerous, the whole inner surface of the disk being ribbed
with them. (See Figs. 74 and 75.) The ovaries follow the length of the
tubes, though they do not extend quite to their extremity, where they
join the circular tube around the margin of the disk; nor do they
start exactly at the point where the tubes diverge from the central
cavity, but a little below it. (Fig. 74.) Each ovary consists of a
long, brownish, flat bag, split along the middle, so closely folded
together that it seems like a flat blade attached along the length of
the tube. Perhaps a better comparison would be to a pea-pod greatly
elongated, with the edges split along their line of juncture, and
attached to a tube of the same length. The ovaries are not perfectly
straight, but slightly waving, as may be seen in Fig. 74, and these
undulations are stronger when the ovaries are crowded with eggs, as is
the case at the time of spawning.
[Illustration: Fig. 75. Zygodactyla seen in profile.]
The large digestive cavity hangs from the centre of the under side of
the disk (Fig. 75), terminating in the proboscis, which, in this kind
of Jelly-fish, is short in proportion to the diameter of the disk,
while the opening of the mouth is very large. (Fig. 74.) It is
unfortunate that a variety of inappropriate names, likely to mislead
rather than aid the unscientific observer, have been applied to
different parts of the Jelly-fish. What we call here digestive cavity,
proboscis, and mouth, are, in fact, parts of one organ. An exceedingly
delicate, transparent, filmy membrane hangs from the under side of the
disk; that membrane forms the outer wall of the digestive cavity,
which it encloses; it narrows toward its lower margin, leaving open
the circular aperture called the mouth; this narrowing of the membrane
is produced by a number of folds in its lower part, while at its
margin these folds spread out to form ruffles around the edge of the
mouth, and these ruffles again extend into the long scalloped fringes
hanging down below.
The motion of these Jelly-fishes is very slow and sluggish. Like all
their kind, they move by the alternate dilatation and contraction of
the disk, but in the Zygodactyla these undulations have a certain
graceful indolence, very unlike the more rapid movements of many of
the Medusæ. It often remains quite motionless for a long time, and
then, if you try to excite it by disturbing the water in the tank, or
by touching it, it heaves a slow, lazy sigh, with the whole body
rising slightly as it does so, and then relapses into its former
inactivity. Indeed, one cannot help being reminded, when watching the
variety in the motions of the different kinds of Jelly-fishes, of the
difference of temperament in human beings. There are the alert and
active ones, ever on the watch, ready to seize the opportunity as it
comes, but missing it sometimes from too great impatience; and the
slow, steady people, with very regular movements, not so quick
perhaps, but as successful in the long run; and the dreamy, indolent
characters, of which the Zygodactyla is one, always floating languidly
about, and rarely surprised into any sudden or abrupt expression. One
would say, too, that they have their aristocratic circles; for there
is a delicate, high-bred grace about some of them quite wanting in the
coarser kinds. The lithe, flexible form of the greyhound is not in
stronger contrast to the heavy, square build of the bull dog, than are
some of the lighter, more frail species of Jelly-fish to the more
solid and clumsy ones. Among these finer kinds we would place the
Tima. (Fig. 76.)
_Tima_. (_Tima formosa_ AG.)
One's vocabulary is soon exhausted in describing the different degrees
of consistency in the substance of Jelly-fishes. Delicate and
transparent as is the Tima, it has yet a certain robustness and
solidity beside the Oceania, described above. In fact, all are
gelatinous, all are more or less transparent, and it is not easy to
describe the various shades of solidity in jelly. Perhaps they may be
more accurately represented by the impression made upon the touch than
upon the sight. If, for instance, you place your hand upon a
Zygodactyla, you feel that you have come in contact with a substance
that has a positive consistency; but if you dip your finger into a
bowl where a Tima is swimming, and touch its disk, you will feel no
difference between it and the water in which it floats, and will not
be aware that you have reached it till the animal shrinks away from
the contact.
[Illustration: Fig. 76. Tima; half natural size.]
[Illustration: Fig. 77. One of the lips of the mouth at the
extremity of the long proboscis; _m_ mouth, _d_ digestive cavity,
_c_ chymiferous tube.]
The adult Tima, represented in Fig. 76, is not more than an inch and a
half or two inches in diameter. Instead of countless tubes diverging
from the digestive cavity to the margin of the disk, as in the
Zygodactyla, there are but four. The digestive cavity in the Tima is
much smaller than in the Zygodactyla, and is placed at the end of the
proboscis, which is long, and hangs down far below the disk. This
removal of the digestive cavity to the extremity of the proboscis
gives to the tubes arising from it a very different and much sharper
curve than they have in the Zygodactyla. In the Tima they start from
the end of the proboscis, as may be seen in the wood-cut (Fig. 76),
and then turn abruptly off, when they arrive at the under surface of
the disk, to reach its margin. The disk has, as usual, its veil and
its fringe of tentacles; the tentacles in the full-grown Tima are
few,--seven in all the four intermediate spaces between the tubes,
with one at the base of each tube, making thirty-two in all. The
ovaries, which are milk-white, follow the line of the tubes, as in the
Zygodactyla, and have very undulating folds when full of eggs. The
tubes meet in the digestive cavity, the margin of which spreads out to
form four ruffled edges that hang down from it. One of these ruffles,
considerably magnified, is represented in Fig. 77. In Fig. 78 we have
a portion of the Hydroid stock from which this Jelly-fish arises, also
greatly magnified. The Tima is very active, yet not abrupt in its
motions; but when in good condition it is constantly moving about,
rising to the surface by the regular pulsations of the disk, or
swimming from side to side, or poising itself quietly in the water,
giving now and then a gentle undulation to keep itself in position.
[Illustration: Fig. 78. Magnified head of Hydrarium of Tima.]
Though not a very frequent visitor of our shores, the appearance of
the Tima is not limited by the seasons, since they are found at all
times of the year. It is a fact, unexplained as yet, that the Tima and
many other Jelly-fishes are never seen except when full grown. What
may be the haunts and habits of these animals from the time of their
hatching till they make their appearance again in the adult condition,
is not known, though it is probable that they remain at the bottom
during this period, and only come to the surface to spawn. This
impression is confirmed by the observations made upon a very young
Cyanea which was kept for a long time in confinement; but a question
of this kind cannot of course be settled by a single experiment.[6]
[Footnote 6: Since the above was written, I have had an opportunity of
learning some additional facts respecting the habits of the young
Cyanea, which may, perhaps, apply to other Jelly-fishes also. Having
occasion to visit the wharves at Provincetown at about four o'clock
one morning, I was surprised to find thousands of the spring brood of
Cyaneæ, hitherto supposed to pass the early period of their existence
wholly in deep water, floating about near the surface. They varied in
size, some being no larger than a three-cent-piece, while others were
from an inch in diameter to three inches. It would seem that they make
their appearance only during the earliest morning hours, for at seven
o'clock, when I returned to the same spot, they had all vanished. It
may be that other young Medusæ have the same habits of early rising,
and that instead of coming to bask in the midday sunshine, like their
elders, they prefer the cooler hours of the dawn. (_A. Agassiz_.)]
_Melicertum_. (_Melicertum campanula_ PÉR. et LES.)
A pretty Medusa, smaller and far more readily obtained than the Tima,
is the Melicertum. (Fig. 80.) Its disk has a yellowish hue, and from
its margin hangs a heavy row of yellow tentacles, while the eight
ovaries (Fig. 79) are of a darker shade of the same color. This little
golden-tinted Jelly-fish, moving through the water with short, quick
throbs, produced by the rapid rise and fall of the disk, is a very
graceful object. Its bright color, made particularly prominent by the
darker undulating lines of the ovaries, which become very marked near
the spawning season, renders it more conspicuous in the water than one
would suppose from its size; for it does not measure more than an inch
in height when full grown. (See Fig. 80.)
[Illustration: Fig. 79. Melicertum campanula seen from above; _m_
mouth, _o o_ ovaries, _t t_ tentacles. (_Agassiz_.)]
_Development of Melicertum and Tima_.
[Illustration: Fig. 80. Melicertum seen in profile; natural size.]
In the Melicertum and Tima we have had the good fortune to trace the
process by which the eggs are changed into Hydroid communities. If any
one has a curiosity to follow for themselves this singular history of
alternate generations, the Melicertum is a good subject for the
experiment, as it thrives well in confinement. After keeping a number
of them in a large glass jar for a couple of days at the time of
spawning, it will be found that the ovaries, which were at first quite
full of eggs, are emptied, and that a number of planulæ; are swimming
about near the bottom of the vessel. After a day or two the outline of
these planulæ, spherical at first, becomes pear-shaped (see Fig. 81),
and presently they attach themselves by the blunt end to the bottom of
the jar. (Fig. 82.) Thus their Hydroid life begins; they elongate
gradually, the horny sheath is formed around them, tentacles arise on
the upper end, short and stunted at first, but tapering rapidly out
into fine flexible feelers, the stem branches, and we have a little
Hydroid community (Fig. 83), upon which, in the course of the
following spring, the reproductive calycles containing the Medusæ buds
will be developed, as in the case of the Eucope and Clytia. The Tima
passes through exactly the same process, though the shape of the
planulæ and the appearance of the young differ from that of the
Melicertum, as may be seen in Fig. 78, where a single head of the Tima
Hydroid, greatly magnified, is represented. By combining the above
observations upon the development of the Hydroids of the Melicertum
and Tima with those previously mentioned upon the young Medusa arising
from reproductive calycles in the Eucope and Clytia, we get a complete
picture of all the changes through which any one of these Hydroid
Medusæ passes, from its Hydroid condition to the moment when it enters
upon an independent existence as a free Jelly-fish.
[Illustration: Fig. 81. Planula of Melicertum; magnified.]
[Illustration: Fig. 82. Cluster of planulæ just attached to the
ground.]
[Illustration: Fig. 83. Young Hydrarium developed from planulæ;
magnified.]
(_Laomedea amphora_ AG.)
The Medusæ of the Campanularians are not all free. On the contrary, in
many of the species they always remain attached to the Hydroid, never
attaining so high a development as the free Medusæ, and withering on
the stem after having laid their eggs. Such is the _Laomedea amphora_,
quite common on all the bridges connecting Boston with the country,
where, on account of the large amount of food brought down from the
sewers by the river, they thrive wonderfully, growing to a great size,
sometimes measuring from a foot to eighteen inches in height.
_Sertularians_.
[Illustration: Fig. 84. Colony of Dynamena pumila; natural size.]
[Illustration: Fig. 85. Magnified portion of Fig. 84.]
The Sertularians form another group of Hydroids closely allied to the
Campanularians, though differing from them in the arrangement of the
sterile Hydræ upon the stem. Among these one of the most numerous is
the Dynamena (_Dynamena pumila_ Lamx., Fig. 84), which hangs its
yellowish fringes from almost every sea-weed above low-water-mark. It
is especially thick and luxuriant on the fronds of our common _Fucus
vesiculosus_. The color is usually of a pale yellow, though sometimes
it is nearly white, and when first taken from the water it has a
glittering look, such as a white frost leaves on a spray of grass.
Fig. 84 represents such a cluster in natural size, while Fig. 85 shows
a piece of the stem highly magnified, with a reproductive calycle
attached to the side of a sterile Hydra stem. Many of these
Sertularian Hydroids assume the most graceful forms, hanging like long
pendent streamers from the Laminaria, or in other instances resembling
miniature trees. One of these tree-like Sertularians (_Dyphasia
rosacea_ Ag.), abundant on all rocks in sheltered places immediately
below low-water-mark, is represented in Fig. 86. In both these
Sertularians the Medusæ wither on the stock, never becoming free. The
free Medusæ of the Sertularians are only known in their adult
condition in a single genus, which is closely allied to Melicertum,
and which is produced from a Hydroid genus called Lafoea. Fig. 87
represents one of these young Sertularian Medusæ (_Lafoea cornuta_
Lamx.).
[Illustration: Fig. 86. Dyphasia rosacea, natural size.]
[Illustration: Fig. 87. Medusa of Lafoea.]
_Tubularians_.
In the Sertularian and Campanularian Hydroids we have found that the
communities consist generally of a large number of small individuals,
so small, indeed, that it is hardly possible at first glance to
distinguish the separate members of these miniature societies. Among
the Tubularians, on the contrary, the communities are usually composed
of a small number of comparatively large individuals; and indeed these
Hydroids may even grow singly, as in the case of the Hybocodon (Fig.
104), which attains several inches in height. There is also another
general feature in which the Tubularians differ from both the other
groups of Hydroids. In the latter, the horny sheath which encloses the
stem extends to form a protecting calycle around the Hydra heads. This
protecting calycle is wanting round the heads of the Tubularians,
though their stems are surrounded by a sheath.
_Sarsia_. (_Coryne mirabilis_ AG.)
[Illustration: Fig. 88. Colony of Coryne; natural size.
(_Agassiz_.)]
[Illustration: Fig. 89. Magnified head of Coryne; _a_ stem, _t_
tentacles, _o_ mouth, _v_ body, _d_ Medusa. (Agassiz.)]
[Illustration: Fig. 90. Free Medusa of Coryne. (_Agassiz_.)]
Among the most common of our Tubularians is a small, mossy Hydroid
(Fig. 88), covering the rocks between tides, in patches of several
feet in diameter. Fig. 89 represents a single head from this little
mossy tuft greatly magnified, in which is seen the medusa bud arising
from the stem by the process already described in the other Hydroids.
In Fig. 90 we have the little Jelly-fish in its adult condition, about
the size of a small walnut, with a wide circular opening, through
which passes the long proboscis, hanging from the under surface of the
disk to a considerable distance below its margin. The four tentacles
are of an immense length when compared to the size of the animal. As a
general thing, the tentacles are less numerous in the Tubularian
Medusæ than in those arising from other Hydroids; they want also the
singular limestone concretions found at the base of the tentacles in
the Campanularian Medusæ. In Fig. 91 we have one of the Tubularian
Medusæ (_Turris vesicaria_ A. Ag.) which lifts a rather larger number
of tentacles than is usual among these Jelly-fishes. We never find the
tentacles multiplying almost indefinitely in them, as in Zygodactyla
and Eucope. The little Jelly-fish described above is known as Sarsia,
while its Hydroid is called Coryne. These names having been given to
the separate phases of its existence before their connection was
understood, and when they were supposed to represent two distinct
animals. They are especially interesting with reference to the history
of Hydroids in general, because they were among the first of these
animals in whom the true relation between the different phases of
their existence was discovered. Lesson named the Sarsia after the
great Norwegian naturalist, Sars, to whom we owe so large a part of
what is at present known respecting this curious subject of alternate
generations.
[Illustration: Fig. 91. Turris vesicaria; natural size.]
_Bougainvillia_. (_Bougainvillia superciliaris_ AG.)
The Bougainvillia (Fig. 92), is one of our most common Jelly-fishes,
frequenting our wharves as well as our sea-shore during the spring. The
tentacles are arranged in four bunches or clusters at the junction of
the radiating tubes with the circular tube, from which they may be seen
extending in every direction whenever these animals remain quietly
suspended in the water,--a favorite attitude with them, and one which
they retain sometimes for days, seeming to make no effort beyond that of
gently playing their tentacles to and fro (Fig. 92). These tentacles are
capable of immense extension, sometimes to ten or fifteen times the
diameter of the bell. The proboscis is not simple as in the Sarsia, but
looks like a yellow urn suspended at its four corners from the
chymiferous tubes. The oral opening is entirely concealed by clusters of
shorter tentacles surrounding the mouth in a close wreath, on which the
eggs are supported. A highly magnified branch of the Hydroid stock from
which this Medusa arises is represented in Fig. 93. There we see the
little Jelly-fishes in different degrees of development on the stem,
while in Figs. 94-97 they are given separately and still more enlarged.
In Fig. 94 the outline of the Jelly-fish is still oval, the proboscis is
but just formed, and the tentacles appear only as round swellings or
knobs. In Fig. 95 a depression has taken place at the upper end,
presently to be an opening, the proboscis is enlarged, and the tentacles
lengthened, but still turned inward. In Fig. 96 the appendages of the
proboscis are quite conspicuous, the tentacles are turned outward, and
the Jelly-fish is almost ready to break from its attachment, having
assumed its ultimate outline. Fig. 97 represents it just after it has
separated from the stem, when it has only two tentacles at each cluster
and simple knobs around the mouth, instead of the complicated branching
tentacles of the adult.
[Illustration: Fig. 92. Bougainvillia; magnified.]
[Illustration: Fig. 93. Hydrarium of Bougainvillia; magnified.]
[Illustration: Figs. 94, 95, 96. Medusæ buds of Fig. 93, in
different degrees of development.]
[Illustration: Fig. 97. Young Medusa just freed from the Hydroid;
magnified.]
_Tubularia_. (_Tubularia Couthouyi_ AG.)
There are several other Tubularians common in our waters which should
not be passed over without mention, although as this little book is by
no means intended as a complete text-book, but rather as a volume of
hints for amateur collectors, we would avoid as much as possible
encumbering it with many names, or with descriptions already given in
more comprehensive works. This Tubularia is interesting, however, from
the fact that the Medusæ buds are never freed from the stem, and do
not develop into full-grown Jelly-fishes, but always remain abortive.
Fig. 98 represents one head of such a Hydroid with the Medusæ buds
pendent from it in a thick cluster, while in Fig. 99 we have a few of
them sufficiently magnified to show that, though presenting the four
chymiferous tubes, they are otherwise exceedingly simple in structure,
as compared with the free Jelly-fishes.
[Illustration: Fig. 98. Tubularia; magnified. (_Agassiz_.)]
[Illustration: Fig. 99. Part of cluster of Medusæ of Fig. 98;
magnified. (_Agassiz_.)]
_Hydractinia_. (_Hydractinia polyclina_ AG.)
This is another Tubularian, covering the surface of rocks in
tide-pools, or attaching itself upon shells inhabited by hermit crabs.
Indeed it was upon these shells that the Hydractinia was first
noticed, and it was long supposed that the wanderings to which the
little colony was thus subjected were necessary for its healthy
development. But subsequent observations have shown that it attaches
itself quite as frequently to the solid rock as to these nomadic
shells. It has a rosy color, and, being very small, it looks, until
one examines it closely, more like a thick red carpet of soft moss,
than like a colony of animals. These communities are distinct in sex,
the fertile individuals in each being either all male or all female.
In Fig. 100 we have a portion of a female colony, representing one
fertile head, in which the buds are crowded with Medusæ; one sterile
head, surrounded by its wreath of tentacles; and still another member
of the society whose office is not fully understood, unless it be that
of a kind of purveyor, catching food for the rest. Fig. 101 represents
the corresponding individuals taken from a male colony. The sex makes
little difference in the appearance of the reproductive heads. All the
individuals of a Hydractinia colony are connected at the base by a
horny network, rising occasionally into points of a conical or
cylindrical shape. This polymorphism among the Tubularians is another
evidence of the relation between the Siphonophoræ, or floating
Hydroids, and the fixed Hydroids.
[Illustration: Fig. 100. Female colony of Hydractinia; _a_ sterile
individual, _b_ fertile individual producing female Medusæ, _c_
fertile individual with globular tentacles without Medusæ, _d_ _e_
_f_ _g_ _h_ _i_ Medusæ in different stages of growth, _o_ mouth
tentacles. (_Agassiz_.)]
[Illustration: Fig. 101. Male colony; _a_ _a_ sterile individuals,
_b_ fertile individuals producing male Medusæ, _d_; _o_ globular
tentacles, _t_ slender tentacles of sterile individual.
(_Agassiz_.)]
_Hybocodon_. (_Hybocodon prolifer_ AG.)
Among our Medusæ derived from a Tubularian stock is the Hybocodon,
viz. the hunchbacked Medusa (Fig. 102), a singular little Jelly-fish,
odd and unsymmetrical in shape, as its name indicates, and interesting
from its relations to one of our floating communities, the Nanomia,
presently to be described. Instead of the evenly proportioned bell of
the ordinary Medusæ, the Hybocodon has a one-sided outline (Fig. 102),
one large tentacle only being fully developed, while the others remain
always abortive, so that the whole weight of the structure is thrown
on one half of the bell. Upon this large tentacle small Jelly-fishes,
similar to the original, are produced by budding, this process going
on till ten or twelve such Jelly-fishes (Fig. 103) may be seen
suspended from the tentacle. Up to this time it has remained connected
with the Hydroid from which it arises, a rather large Tubularian,
usually growing singly (Fig. 104), and of a deep orange-red in color.
But at this stage of its existence it frees itself, and leads an
independent life hereafter, swimming about with a quick, darting
motion. In the account of the Nanomia, the homology between its scale,
or abortive Medusa, and the Hybocodon, is traced in detail, and I need
only allude to it here. Though this Medusa is so peculiar in
appearance, the Tubularian from which it is derived is very like the
_Tubularia Couthouyi_, already described. This is one of the instances
before alluded to, in which closely allied forms give rise to very
dissimilar ones, or, as in many cases, the very reverse of this takes
place, and closely allied forms arise from very dissimilar ones.
[Illustration: Fig. 102. Unsymmetrical free Medusa of Hybocodon;
_r_ _o_ chymiferous tubes, _v_ proboscis, _s_ circular tube, _m_
young Medusæ at base of long tentacle _t_. (_Agassiz_.)]
[Illustration: Fig. 103. Medusa bud of Hybocodon; _a_ base of
attachment, _o_ proboscis, _c_ circular tube, _d_ young Medusæ at
base of long tentacle _t_. (_Agassiz_.)]
[Illustration: Fig. 104. Single head of Hybocodon Hydroid; _a_
stem, _d_ Medusæ buds, _o_ tentacles round mouth. (_Agassiz_.)]
_Dysmorphosa_. (_Dysmorphosa fulgurans_ A. AG.)
Besides the budding at the base of the tentacle, as in Hybocodon, we
find another mode of development among Hydroid Medusæ, viz. that of
budding from the proboscis. One of our most common little
Jelly-fishes, the Dysmorphosa (Fig. 105), to which we owe the
occasional blue phosphorescence of the sea, so brilliant at times,
buds in this manner. Fig. 105 represents an adult Dysmorphosa, on the
proboscis of which may be seen three small buds in different stages of
development. In Fig. 106 the proboscis is more enlarged, showing one
of the little Jelly-fishes similar to the parent, just ready to drop
off. We need not wonder at the immense number of these animals, with
which the sea actually swarms at times, when we know that as fast as
they are dropped, and it takes but a few days to complete their
development, they each begin the same process; so that in the course
of a week or ten days one such Medusa, supposing it to have produced
six buds only, will have given rise to forty-two Jelly-fishes,
thirty-six of which may be equally prolific in the same short period.
These Medusæ budding thus, and swimming about, carrying their young
with them, bear such a close resemblance to the floating communities
of Hydroids formerly known as Siphonophoræ, that did we not know that
some of them arise from Tubularians, it would be natural to associate
them with the Siphonophoræ.
[Illustration: Fig. 105. Dysmorphosa seen in profile; magnified.]
[Illustration: Fig. 106. Magnified proboscis of Dysmorphosa with
young Medusæ budding from it.]
_Nanomia_. (_Nanomia cara_ A. AG.)
The Nanomia (Fig. 115), our free floating Hydroid, consists, when
first formed, of a single Hydra containing an oblong oil bubble (Fig.
107). The whole organisation of such a Hydra is limited to a simple
digestive cavity; it has, in fact, but one organ, and one function,
and consists of an alimentary sac resembling the proboscis of a Medusa
(Fig. 107); the oil bubble is separated from it by a transverse
partition, and has no connection with the cavity. Presently, between
the oil bubble and the cavity arise a number of buds of various
character (Fig. 108), which we will describe one by one, beginning
with those nearest the oil bubble, since these upper members of the
little swimming community bear a very important part in its history.
The infant community (Fig. 108) passes rapidly into the stage
represented in Fig. 109, and then through all the stages intermediate
between this and the adult, shown in its natural size in Fig. 115. The
upper buds enlarge gradually, and soon take upon themselves a perfect
Medusa structure (Fig. 110), with the exception of the proboscis, the
absence of which is easily understood, when we find that these Medusæ,
serve the purpose of locomotion only, having no share in the function
of feeding the community, so that a digestive apparatus would be quite
superfluous for them. In every other respect they are perfect Medusæ,
attached to the Hydra as the Medusa buds always are when first formed,
having the (four) chymiferous tubes, characteristic of all Hydroid
Medusæ, radiating from the centre to the periphery; two of these tubes
are very winding, as may be seen in Fig. 110, while the other pair are
straight. The Medusæ themselves are heart-shaped in form, depressed at
the centre of the upper surface, and bulging on either side into
wing-like expansions, where they join the stem. These expansions
interlock with one another, crossing nearly at right angles. The
Medusæ-like buds are the swimming bells; by their contractions,
alternately taking in and throwing out the water, they impel the whole
community forward, so that it seems rather to move like one animal,
than like a combination of individuals.
[Illustration: Fig. 107. Young Nanomia; magnified.]
[Illustration: Fig, 108. Young Nanomia with rudimentary Medusæ.]
[Illustration: Fig. 109. Young Nanomia, older than Fig. 108.]
[Illustration: Fig. 110. Heart-shaped swimming bell of Nanomia;
magnified.]
Besides these locomotive members, the community contains three kinds
of Hydræ arising as buds from the primitive Hydra below the swimming
bells, the latter remaining always nearest the oil bubble at the top,
while the first Hydra, the founder of the community, in proportion as
the new individuals are added, is gradually pushed downward, and
remains always at the end of the string, the stem of which is formed
by the elongated neck of the primitive Hydra. All the three sets of
Hydræ have certain features in common, while they have other
distinguishing characteristics marking them as distinct individuals.
They are all accompanied by triangular shields (Fig. 111), arising
with them at the same point on the parent stem, and all are furnished
with tentacles hanging down from the summit of the Hydra at the side
opposite the shield. These facts are important to remember, since we
shall presently perceive, upon analyzing their parts, that these Hydræ
have a close homology to the Hybocodon. The tentacles differ in
structure as well as in number for each kind of Hydra. Having shown in
what characters they agree, let us now take each set individually, and
see what differences they present.
[Illustration: Fig. 111. Cluster of Medusæ with tentacles having
pendent knobs.]
In the first set which we will examine the Hydra is open-mouthed. Like
the original Hydra, it is only a digestive tube, similar in all
respects to the proboscis of a Medusa-disk. Its only function is that
of feeding, and it shows a laudable fidelity to its calling, being
very constantly and earnestly engaged in the work. Let us add,
however, that in this instance the occupation is not a wholly selfish
one, since the cavity of every Hydra communicates with that of the
stem, and the food taken in at these over-gaping mouths, is at once
circulated through all parts of the community, with the exception of
the oil bubble, from which it is excluded by the transverse partition
dividing it from all the lower members of the stock. The shields share
in this general nourishment of the compound body by means of
chymiferous tubes extending toward the outer surface, and opening into
the cavity of the stem. The mouth of this Hydra is very flexible (Fig.
111), expanding and contracting at the will of the animal, and
sometimes acting as a sucker, fastening itself, leech-like, on the
object from which it seeks to draw its sustenance. (See Fig. 111.) The
tentacles attached to this set of Hydræ are exceedingly long and
delicate. They arise in a cluster at the upper and inner edge of the
Hydra, just at its point of juncture with the stem, and being
extremely flexible and contractile, their long tendril-like sprays are
thrown out in an endless variety of attitudes. (See Fig. 115.) Along
the whole length of this kind of tentacle are attached little pendent
knobs at even distances; Fig. 112 represents such a knob greatly
magnified, and absolutely paved with lasso-cells, the inner and
smaller ones being surrounded by a row of larger ones.
[Illustration: Fig. 112. Magnified pendent knob.]
[Illustration: Fig. 113. Medusa with corkscrew shaped tentacles.]
The second set of Hydræ (Fig. 113), are also open-mouthed,
corresponding with those described above, in everything except the
tentacles, which are both shorter and thicker, and are coiled in a
corkscrew-like spiral. These are thickly studded for their whole
length with lasso-cells. (See Fig. 113.)
In the third and last set of Hydræ (Fig. 114), the mouth is closed;
they have, therefore, no share in feeding the community, but receive
their nourishment from the cavity of the stem into which they open.
They differ also from the others in having a single tentacle instead
of a cluster, and on this tentacle the lasso-cells are scattered at
uneven distances (Fig. 114). The special function of these closed
Hydræ is yet to be explained; they have oil bubbles at their upper end
(see Fig. 111, the top Hydra), and though we have never seen them drop
off, it seems natural to suppose that they do separate from the parent
stock, and found new communities similar to those from which they
arise.
[Illustration: Fig. 114. Medusa with a simple thread-like
tentacle.]
The intricate story of this singular compound existence does not end
here. There is still another set of individuals whose share in
maintaining the life of the community is by no means the least
important. Little bunches of buds, of a different character from any
described above, may be seen at certain distances along the lower part
of the stem. These are the reproductive individuals. They are clusters
of imperfect sexual Medusæ, resembling the rudimentary Medusæ of
Tubularia (Fig. 99), which are never freed from the parent stem, but
discharge their contents at the breeding season. Like many other
compound Hydroids, the sexes are never combined, in one of these
communities; they are always either male or female, and as those with
female buds have not yet been observed, we can only judge by inference
of their probable character. Front what is already known, however, of
Hydroid communities of a like description, we suppose that the process
of reproduction must be the same in these, and that the female stocks
of Nanomia give birth to small Jelly-fishes, the eggs of which become
oil bubbles, similar to that with which our little community began.
(Fig. 108.)
[Illustration: Fig. 115. Adult Nanomia, natural size, at rest.]
By the time all these individuals have been added along the length of
the stem, the stem itself has grown to be about three inches long
(Fig. 115), though the tentacles hanging from the various members of
the community give to the whole an appearance of much greater length.
The motion of this little string of living beings is most graceful.
The oil bubble (Fig. 116) at the upper end is their float; the
swimming bells immediately below it (Fig. 110), by the convulsive
contractions of which they move along, are their oars. The water is
not taken in and expelled again by all the bells at once, but first
from all the bells on one side, beginning at the lower one, and then
from all those on the opposite side, beginning also at the lower one;
this alternate action gives to their movements a swinging, swaying
character, expressive of the utmost freedom and grace. Whether such a
little community darts with a lightning-like speed through the water,
or floats quietly up and down, for its movements are both rapid and
gentle, it always sways in this way from side to side. Its beauty is
increased by the spots of bright red scattered along the length of the
stock at the base and tips of the Hydræ, as well as upon the
tentacles. The movements and attitudes of the tentacles are most
various. Sometimes they shoot them out in straight lines on either
side, and then the aspect of the whole thing reminds one of a tiny
chandelier in which the coral drops make the pendants, or they may be
caught up in a succession of loops or floating in long streamers;
indeed, there is no end to the fantastic forms they assume, ever
astonishing you by some new combination of curves. The prevailing hue
of the whole community is rosy, with the exception of the oil bubble
or float, which looks a bright garnet color when seen in certain
lights.
[Illustration: Fig. 116. Oil float of Nanomia; greatly magnified.]
Let us now compare one of the Hydræ hanging from the stem (Fig. 113)
with the Hybocodon (Fig. 102). The reader will remember the
unsymmetrical bell of this singular Medusa, one half of its disk more
largely developed than the other, with the proboscis hanging from the
centre, and the cluster of tentacles from one side. Let us now split
the bell so as to divide it in two halves with the proboscis hanging
between them; next enlarge the side where there are no tentacles, and
give it a triangular outline; then contract the opposite side so as to
draw up the cluster of tentacles to meet the base of the proboscis,
and what have we? The proboscis now corresponds to the Hydra of our
Nanomia, with the cluster of tentacles attached to its upper edge
(Fig. 113), while the enlarged half of the bell represents the shield.
If this homology be correct it shows that the Nanomia is not, as some
naturalists have supposed all the Siphonophores to be, a single
animal, its different parts being a mere collection of organs endowed
with special functions, as feeding, locomotion, reproduction, &c., but
that it is indeed a community of distinct individuals corresponding
exactly to the polymorphous Hydroids, whose stocks are attached, such
as Hydractinia, and differing from them only in being free and
floating.
The homologies of the Siphonophoræ or floating Hydroids, with many of
the fixed Hydroids, is perhaps more striking when we compare the
earlier stages of their growth. Suppose, for instance, that the
planula of our Melicertum (See Fig. 81) should undergo its development
without becoming attached to the ground,--what should we then have? A
floating community (Fig. 83), including on the same stock like the
Nanomia, both sterile and fertile Hydræ, from the latter of which
Medusæ bells are developed. The little Hydractinia community (Fig.
100), in which we have no less than four distinct kinds of
individuals, each performing a definite distinct function, affords a
still better comparison.
_Physalia_. (_Physalia Arethusa_ TIL.)
[Illustration: Fig. 117. Physalia; _a_ _b_ air sac with crest _c_,
_m_ bunches of individuals, _n_ central tentacles, _t_ _t_
expanded tentacles. (_Agassiz_.)]
Among the most beautiful of the Siphonophores, is the well-known
Physalia or Portuguese man-of-war, represented in Fig. 117. The float
above is a sort of crested sac or bladder, while the long streamers
below consist of a number of individuals corresponding in their nature
and functions to those composing a Hydroid community. Among them are
the fertile and sterile Hydræ (Fig. 118), the feeders and Medusæ bells
(Fig. 119). The Physalia properly belongs to tropical waters, but
sometimes floats northward, in the warm current of the Gulf Stream,
and is stranded on Cape Cod. When found so far from their home,
however, they have usually lost much of their vividness of color; to
judge of their beauty one should see them in the Gulf of Mexico,
sailing along with their brilliant float fully expanded, their crest
raised, and their long tentacles trailing after them.
[Illustration: Fig. 118. Bunch of Hydræ; _a_ base of attachment,
_b_ _b_ _b_ single Hydræ, _c_ _c_ tentacles. (_Agassiz_.)]
[Illustration: Fig. 119. Bunch of Hydræ; cluster of Medusæ; _b_
_b_ Hydræ with tentacles, _c_ _d_ bunches of Medusæ (_Agassiz_.)]
_Velella_. (_Velella mutica_ BOSC.)
Another very beautiful floating Hydroid, occasionally caught in our
waters, though its home is also far to the south, is the Velella (Fig.
120). It is bright blue in color, and in form not unlike a little flat
boat with an upright sail. Its Medusa (Fig. 121) resembles so much
that of some of our Tubularians, that it has actually been removed on
this account from the old group of Siphonophoræ, and placed next the
Tubularians; another evidence of the close affinity between the former
and the Hydroids.
[Illustration: Fig. 120. Velella; _m_ so-called mouth, _a_
tentacles. (_Agassiz_.)]
[Illustration: Fig. 121. Free Medusa of Velella; _a_ proboscis,
_b_ chymiferous tube, _c_ circular tube. (_Agassiz_.)]
MODE OF CATCHING JELLY-FISHES.
Not the least attractive feature in the study of these animals, is the
mode of catching them. We will suppose it to be a warm, still morning
at Nahant, in the last week of August, with a breath of autumn in the
haze that softens the outlines of the opposite shore, and makes the
horizon line a little dim. It is about eleven o'clock, for few of the
Jelly-fishes are early risers; they like the warm sun, and at an
earlier hour they are not to be found very near the surface. The sea
is white and glassy, with a slight swell but no ripple, and seems
almost motionless as we put off in a dory from the beach near
Saunders's Ledge. We are provided with two buckets, one for the larger
Jelly-fishes, the Zygodactyla, Aurelia, &c., the other for the smaller
fry, such as the various kinds of Ctenophoræ, the Tima, Melicertum,
&c. Beside these, we have two nets and glass bowls, in which to take
up the more fragile creatures that cannot bear rough handling. A bump
or two on the stones before we are fairly launched, a shove of the oar
to keep the boat well out from the rocks along which we skirt for a
moment, and now we are off. We pull around the point to our left and
turn toward the Ledge, filling our buckets as we go. Now we are
crossing the shallows that make the channel between the inner and
outer rocks of Saunders's Ledge. Look down,--how clear the water is
and how lovely the sea-weeds, above which we are floating, dark brown
and purple fronds of the Ulvæ, and the long blades of the Laminaria
with mossy green tufts between. As we issue from this narrow passage
we must be on the watch, for the tide is rising, and may come laden
with treasures, as it sweeps through it. A sudden cry from the oarsman
at the bow, not of rocks or breakers ahead, but of "A new Jelly-fish
astern!" The quick eye of the naturalist of the party pronounces it
unknown to zoölogists, un-described by any scientific pen. Now what
excitement! "Out with the net!--we have passed him! he has gone down!
no, there he is again! back us a bit." Here he is floating close by
us; now he is within the circle of the net, but he is too delicate to
be caught safely in that way, so, while one of us moves the net gently
about, to keep him within the space enclosed by it, another slips the
glass bowl under him, lifts it quickly, and there is a general
exclamation of triumph and delight,--we have him. And now we look more
closely; yes, decidedly he is a novelty as well as a beauty. (See Fig.
122, _Ptychogena lactea_ A. Ag.) Those white mossy tufts for ovaries
are unlike anything we have found before (Fig. 123), and not
represented in any published figures of Jelly-fishes. We float about
here for a while, hoping to find more of the same kind, but no others
make their appearance, and we keep on our way to East Point, where
there is a capital fishing ground for Medusæ of all sorts. Here two
currents meet, and the Jelly-fishes are stranded as it were along the
line of juncture, able to move neither one way nor the other. At this
spot the sea actually swarms with life; one cannot dip the net into
the water without bringing up Pleurobrachia, Bolina, Idyia,
Melicertum, &c., while the larger Zygodactyla and Aurelia float about
the boat in numbers. These large Jelly-fishes produce a singular
effect as one sees them at some depth beneath the water; the Aureliæ,
especially, with their large white disks, look like pale phantoms
wandering about far below the surface; but they constantly float
upward, and if not too far out of reach, one may bring them up by
stirring the water under them with the end of the oar.
[Illustration: Fig. 122. Ptychogena, natural size.]
[Illustration: Fig. 123. Ovary of Ptychogena; magnified.]
When we have passed an hour or so floating about just beyond East
Point, and have nearly filled our buckets with Jelly-fishes of all
sizes and descriptions, we turn and row homeward. The buckets look
very pretty as they stand in the bottom of the boat with the sunshine
lighting up their living contents. The Idyia glitters and sparkles
with ever-changing hues, the Pleurobrachiæ dart about, trailing their
long graceful tentacles after them, the golden Melicerta are kept in
constant motion by their quick, sudden contractions, and the delicate
transparent Tima floats among them all, not the less beautiful because
so colorless. There is an unfortunate Idyia, who, by some mistake, has
got into the wrong bucket with the larger Jelly-fish, where a
Zygodactyla has entangled it among his tentacles and is quietly
breakfasting upon it.
During our row the tide has been rising, and as we near the channel of
Saunders's Ledge, it is running through more strongly than before, and
at the entrance of the shallows a pleasant surprise is prepared us; no
less than half a dozen of our new friends (the Ptychogena as he has
been baptized), come to look for their lost companion perhaps, await
us there, and are presently added to our spoils. We reach the shore
heavily laden with the fruits of our morning's excursion.
The most interesting part of the work for the naturalist is still to
come. On our return to the Laboratory, the contents of the buckets are
poured into separate glass bowls and jars; holding them up against the
light, we can see which are our best and rarest specimens; these we
dip out in glass cups and place by themselves. If any small specimens
are swimming about at the bottom of the jar, and refuse to come within
our reach, there is a very simple mode of catching them. Dip a glass
tube into the water, keeping the upper end closed with your finger,
and sink it till the lower end is just above the animal you want to
entrap; then lift your finger, and as the air rushes out the water
rushes in, bringing with it the little creature you are trying to
catch. When the specimens are well assorted, the microscope is taken
out, and the rest of the day is spent in studying the new
Jelly-fishes, recording the results, making notes, drawings, &c.
Still more attractive than the rows by day are the night expeditions
in search of Jelly-fishes. For this object we must choose a quiet
night, for they will not come to the surface if the water is troubled.
Nature has her culminating hours, and she brings us now and then a day
or night on which she seems to have lavished all her treasures. It was
on such a rare evening, at the close of the summer of 1862, that we
rowed over the same course by Saunders's Ledge and East Point
described above. The August moon was at her full, the sky was without
a cloud, and we floated on a silver sea; pale streamers of the aurora
quivered in the north, and notwithstanding the brilliancy of the moon,
they too cast their faint reflection in the ocean. We rowed quietly
along past the Ledge, past Castle Rock, the still surface of the water
unbroken, except by the dip of the oars and the ripple of the boat,
till we reached the line off East Point, where the Jelly-fishes are
always most abundant, if they are to be found at all. Now dip the net
into the water. What genie under the sea has wrought this wonderful
change? Our dirty, torn old net is suddenly turned to a web of gold,
and as we lift it from the water heavy rills of molten metal seem to
flow down its sides and collect in a glowing mass at the bottom. The
truth is, the Jelly-fishes, so sparkling and brilliant in the
sunshine, have a still lovelier light of their own at night; they give
out a greenish golden light as brilliant as that of the brightest
glow-worm, and on a calm summer night, at the spawning season, when
they come to the surface in swarms, if you do but dip your hand into
the water it breaks into sparkling drops beneath your touch. There are
no more beautiful phosphorescent animals in the sea than the Medusæ;
it would seem that the expression, "rills of molten metal" could
hardly apply to anything so impalpable as a Jelly-fish, but, although
so delicate in structure, their gelatinous disks give them a weight
and substance; and at night, when their transparency is not perceived,
and their whole mass is aglow with phosphorescent light, they truly
have an appearance of solidity which is most striking, when they are
lifted out of the water and flow down the sides of the net.
The various kinds present very different aspects; wherever the larger
Aureliæ and Zygodactylæ float to the surface, they bring with them a
dim spreading halo of light, the smaller Ctenophoræ become little
shining spheres, while a thousand lesser creatures add their tiny
lamps to the illumination of the ocean; for this so-called
phosphorescence of the sea is by no means due to the Jelly-fishes
alone, but is also produced by many other animals, differing in the
color as well as the intensity of their light, and it is a curious
fact that they seem to take possession of the field by turns. You may
row over the same course, which a few nights since glowed with a
greenish golden light wherever the surface of the water was disturbed,
and though equally brilliant, the phosphorescence has now a pure white
light. On such an evening, be quite sure that when you empty your
buckets on your return and examine their contents you will find that
the larger part of your treasures are small crustacea (little
shrimps). Of course there will be other phosphorescent creatures,
Jelly-fishes, &c., among them, but the predominant color is given by
these little crustacea. On another evening the light will have a
bluish tint, and then the phosphorescence is principally due to the
Dysmorphosa (Fig. 105).
Notwithstanding the beauty of a moonlight row, if you would see the
phosphorescence to greatest advantage you must choose a dark night,
when the motion of your boat sets the sea on fire around you, and a
long undulating wave of light rolls off from your oar as you lift it
from the water. On a brilliant evening this effect is lost in a great
degree, and it is not until you dip your net fairly under the moonlit
surface of the sea, that you are aware how full of life it is.
Occasionally one is tempted out by the brilliancy of the
phosphorescence, when the clouds are so thick that water, sky, and
land become one indiscriminate mass of black, and the line of rocks
can be discerned only by the vivid flash of greenish golden light,
when the breakers dash against them. At such times there is something
wild and weird in the whole scene, which at once fascinates and
appalls the imagination; one seems to be rocking above a volcano, for
the surface around is intensely black, except where fitful flashes or
broad waves of light break from the water under the motion of the boat
or the stroke of the oars. It was on a night like this, when the
phosphorescence was unusually brilliant, and the sea as black as ink,
the surf breaking heavily and girdling the rocky shore with a wall of
fire, that our collector was so fortunate as to find in the rich
harvest he brought home the entirely new and exceedingly pretty little
floating Hydroid, described under the name of Nanomia (Fig. 115). It
was in its very infancy (Fig. 108), a mere bubble, not yet possessed
of the various appendages which eventually make up its complex
structure; but it was nevertheless very important to have seen it in
this early stage of its existence, since, when a few full-grown
specimens were found in the autumn, which lived for some days in
confinement and quietly allowed their portraits to be taken (see Fig.
115), it was easy to connect the adult animal with the younger phase
of its own life and thus make a complete history.
Marine phosphorescence is no new topic, and we have dwelt too long,
perhaps, upon a phenomenon that every voyager has seen, and many have
described. Its effect is very different, when seen from the deck of a
vessel, from its appearance as one floats through its midst,
distinguishing the very creatures that produce it, and any account of
the Medusæ which did not include this most characteristic feature
would be incomplete.
* * * * *
ECHINODERMS.
Our illustrations and descriptions of Echinoderms are scanty in
comparison to those of the preceding class; for while, in consequence,
perhaps, of the combined influence of the Gulf Stream and the cold
arctic current on the New England shore, Acalephs are largely
represented in our waters, our marine fauna is meagre in Echinoderms.
But although we have few varieties, those which do establish
themselves on a coast seemingly so ungenial for others of their kind,
such as the Echinus, and our common Star-fish, for instance, thrive
well and are very abundant. The class of Echinoderms includes five
orders, viz. CRINOIDS, OPHIURANS, STAR-FISHES, SEA-URCHINS, and
HOLOTHURIANS. The animals composing these orders differ so widely in
appearance that it was very long before their true relations were
detected, and it was seen that all their external differences were
united under a common plan. Let us compare, for instance, the
worm-like Holothurians (Figs. 124, 126, 127) with all the host of
Star-fishes (Figs. 142, 146, 147) and Sea-urchins (Figs. 131, 139), or
compare the radiating form of the Star-fish, its arms spreading in
every direction, with the close spherical outline of the Sea-urchin,
or the Crinoid floating at the end of a stem (Fig. 152) with either of
these, and we shall cease to wonder that naturalists failed to find at
once a unity of idea under all these varieties of execution. And yet
the fundamental structure of the class of Echinoderms is represented
as distinctly by any one of its five orders as by any other, and is
absolutely identical in all. They differ only by trifling
modifications of development.
In Echinoderms as a class, the body presents three regions differing
in structure, and on the greater or less development of these regions
or systems, as we may call them, their chief differences are based.
Take, for instance, the dorsal system, the nature of which is
explained by the name, indicating of course the back of the animal,
though it does not necessarily imply the upper side of the body, since
some of the Echinoderms, as the stemmed Crinoids, for example, carry
the dorsal side downward, while the Star-fishes and Sea-urchins carry
it upward, and the Holothurians, moving with the mouth forward, have
the dorsal system at the opposite end of the body. Whatever the
natural attitude of the animal, however, and the consequent position
of the dorsal region, it exists alike in all the five orders, though
it has not the same extent and importance in each. But in all it is
made up of similar parts, bears the same relation to the rest of the
body, has the same share in the general economy of the animal. And
though when we compare the spreading back of a Star-fish with the
small area on the top of a Sea-urchin, where all the zones unite, we
may not at once see the correspondence between them, yet a careful
comparison of all their structural details shows that they are both
built with the same elements and represent the same region, though it
is stretched to the utmost in the one case, and greatly contracted in
the other.
This being true of the dorsal system, let us look at another equally
important structural feature in this class. All Echinoderms have
locomotive organs peculiar to themselves, a kind of suckers which may
be more or less numerous, larger or smaller, in different species, but
are always appendages of the same character. These are variously
distributed over the body, but always with a certain regularity
occupying definite spaces, shown by investigation to be homologous in
all. For instance, the rays of the Star-fish correspond in every
detail on their under side, along which the locomotive suckers run,
with the zones on the Sea-urchin, from end to end of which the suckers
are arranged; and the same is equally true of the distribution of the
suckers on the Holothurians, Ophiurans, and Crinoids, though, as most
persons are less familiar with these orders than with the other two,
it might not be so easy to point out the coincidence to our readers.
These suckers are called the ambulacra, the lines along which they run
are called the ambulacral rows or zones, while the system of
locomotion as a whole is known as the ambulacral system. Since these
organs are thus regularly distributed over the body in distinct zones
or rows, it follows that the latter must be divided by intervening
spaces. These intervals are called the interambulacral spaces; but
while in some orders they are occupied by larger plates and prominent
spines, as in the Sea-urchin and Star-fish, in others they are either
comparatively insignificant or completely suppressed, as in the
Crinoids and Ophiurans. Such are the three regions or systems which by
their greater or less development introduce an almost infinite variety
of combinations into this highest class of Radiates. It may not be
amiss before proceeding further to compare the five orders with
reference to this point, and see which of these three systems has the
preponderance in each one.
Taking the orders in their rank and beginning with the lowest, we find
in the Crinoids that the dorsal system preponderates, being composed
of highly complicated plates, and developed to such a degree as to
form in many instances a stem by which the animal is attached to the
ground, while the ambulacral system is limited to a comparatively
small area, and the interambulacral system is wanting. The order of
Crinoids has diminished so much in modern geological times that we
must consult its fossil forms in order to understand fully the
peculiar adaptation of the Echinoderm plan in this group.
In the Ophiurans, the dorsal system is still large, and though it no
longer stretches out to form a stem, it folds over on the under side
of the animal so as to enclose entirely the ambulacral system, forming
a kind of shield for the arms. Here also the interambulacral system is
wanting.
In the Star-fishes the dorsal system encroaches less upon the
structure of the animal. The back and oral side here correspond
exactly in size, and though the flat leathery upper surface of the
animal, covered with spines, serves as a protection to the delicate
ambulacral suckers which find their way between the rows of small
plates along the under side of the arms, yet it does not enfold them
as in the Ophiurans. On the contrary, in the Star-fishes the
ambulacral rows are protected on either side by a row of the so-called
interambulacral plates, through which no suckers pass.
In the Sea-urchin, the dorsal system is contracted to a minimum,
forming a small area on the top of the animal, the rows of
interambulacral plates which are separated and lie on either side of
the ambulacra in the Star-fish being united in the Sea-urchin, and
both the ambulacral and the interambulacral systems bent upward,
meeting in the small dorsal area above, so as to form a spherical
outline. Here the ambulacral and interambulacral systems have taken a
great preponderance over the dorsal system, and the same is the case
with the Holothurians, in which the same structure is greatly
elongated, the dorsal system being thus pushed out as it were to the
end of a cylinder, while the ambulacral and interambulacral systems
run along its whole length. All Echinoderms without exception have
ambulacral tubes, even though in some there are no external ambulacral
suckers connected with them.
There is one organ peculiar to the class of Echinoderms, the general
structure of which may be described here, since it is common to them
all, with the exception of the Crinoids, the anatomy of which is,
however, so imperfectly understood, that we are hardly justified in
assuming that it does not exist even in that order. This organ is
known as the madreporic body; it is a small sieve or limestone filter
opening into a tube or canal; by means of this tube, which connects
with the ambulacral system, the water from without, first filtered
through the madreporic body and thus freed from any impurities, is
conveyed to the ambulacra. In the more detailed account of the
different orders we shall see what is the position of this singular
organ in each group, and how it is adapted in them all to their
special structure. The development of Echinoderms forms one of the
most wonderful chapters in the annals of Natural History. Marvellous
as is the embryonic history of the Acalephs, including all the
different aspects they assume in the cycle of their growth, it is
thrown into the shade by the transformations which Echinoderms undergo
before assuming their adult condition. This singular mode of
development, although it has features recalling the development of
Jelly-fishes from Hydroids, is nevertheless entirely distinct from it,
and is known only in the class of Echinoderms. As the whole story is
given at length in the chapter on the embryology of the Echinoderms,
we need only allude to it here in general terms. We owe the discovery
of this remarkable process to Johannes Müller, one of the greatest
anatomists of this century.
* * * * *
HOLOTHURIANS.
_Synapta_. (_Synapta tenuis_ AYRES.)
[Illustration: Fig. 124. Synapta, natural size.]
This is one of the most curious of the Holothurians, and easily
observed on account of its transparency, which allows us to see its
internal structure. It has a long cylindrical body (Fig. 124) along
the length of which run the five rows of ambulacra, which are in this
instance closed tubes without any projecting suckers or locomotive
organs of any kind attached to them, so that the name is retained only
on account of their correspondence in position, and not from any
similarity of function to the ambulacra in Star-fishes and
Sea-urchins. But though the ambulacra in Synapta are in fact mere
water-tubes like the vertical tubes in the Ctenophoræ, by means of
which the water, first filtered through the madreporic body,
circulates along the skin, they are as organs perfectly homologous
with the ambulacra in all other Echinoderms. The mouth has a circular
tube around the aperture, and a wreath of branching tentacles
encircling it. The habits of these animals are singular. They live in
very coarse mud, but they surround themselves with a thin envelope of
finer sand, which they form by selecting the small particles with
their tentacles, and making a ring around their anterior extremity.
This ring they then push down along the length of the body, and
continue this process, adding ring after ring, till they have entirely
encircled themselves with a sand tube. They move the rings down partly
by means of contractions of the body, but also by the aid of
innumerable appendages over the whole surface. To the naked eye these
appendages appear like little specks on the skin; but under the
microscope they are seen to be warts projecting from the surface, each
one containing a little anchor with the arms turned upward (Fig. 125).
Around the mouth these warts are larger, but do not contain any
anchors. It will be seen hereafter that these appendages are
homologous with certain organs in other Holothurians, the warts with
the anchors corresponding to the limestone pavement covering or
partially covering the surface of the Cuvieria, for instance, while
those without anchors correspond to the so-called false ambulacra in
Pentacta. By means of these appendages, though aided also by the
contractions of the body, the Synaptæ move through the mud and collect
around themselves the sand tube in which they are encased. Their food
is very coarse for animals so delicate in structure. When completely
empty of food they are white, perfectly transparent, and the spiral
tube forming the digestive cavity may be seen wound up and hanging
loosely in the centre for the whole length of the body. In such a
condition it is of a pale yellow color. But look at one that is gorged
with food. The whole length of the alimentary canal is then crowded
with sand, pebbles, and shells, distinctly seen through the
transparent skin, and giving a dark gray color to the whole body. They
swallow the sand for the sake of the nutritious substance it contains,
and having assimilated and digested this, they then eject the harder
materials. The motion of the body in consequence of its contractions
is much like that of leeches, and on this account these Synaptæ were
long supposed to be a transition type between the Radiates and worms.
The body grows to a great length, often half a yard and more, but
constantly drops large portions from its posterior part, by means of
its own contractions, or breaks itself up by the expulsion of the
intestines, which are very readily cast out. The tentacles are hollow,
consisting of a central rib with branches from either side. In the
Synaptæ, as in all the Holothurians, the madreporic body is placed
near the mouth, between two of the ambulacra, and opposite the fifth
or odd one. The tube, connecting with the central tube around the
mouth, by means of which it communicates with the ambulacral tubes, is
very short.
[Illustration: Fig. 125. Anchor of Synapta; _a_ anchor, _w_ plate
upon which anchor is attached; greatly magnified.]
_Caudina_. (_Caudina arenata_ STIMPS.)
[Illustration: Fig. 126. Caudina arenata; natural size.]
Several other Holothurians are frequently met with on our shores.
Among them is the _Caudina arenata_ (Fig. 126), a small Holothurian,
yellowish in color, and thick in texture, by no means so pretty as the
white transparent Synapta; the tentacles are short, resembling a crown
of cloves around the mouth. It lives in the sand, and may be found in
great numbers on the sandy beaches after a storm.
_Cuvieria_. (_Cuvieria squamata_ D. & K.)
[Illustration: Fig. 127. Cuvieria; natural size.]
The Holothurian of our coast, excelling all the rest in beauty, is the
Cuvieria. (Fig. 127.) As it lies on the sand, a solid red lump, with
neither grace of form nor beauty of color, even the vividness of its
tint growing dull and dead when it is removed from its native element,
certainly no one could suspect that it possessed any hidden charm; but
place it in a glass bowl with fresh sea-water; the dull red changes to
deep vivid crimson, the tentacles creep out (Fig. 127) softly, and
slowly, till the mouth is surrounded by a spreading wreath, comparable
for richness of tint, and for delicate tracery, to the most beautiful
sea-weeds. These tentacles, when fully expanded, are as long as the
body itself. A limestone pavement composed of numerous pieces covers
almost the whole surface of the animal; this apparatus corresponds, as
we have already mentioned, to the warts containing anchors in the
Synapta; but in the latter, the limestone particles are smaller,
whereas in the Cuvieria they are developed to a remarkable extent.
This animal is very sluggish, the ambulacral suckers, found only on
three of the tubes, being arranged in such a way as to form a sort of
sole on which they creep; the sole is tough and leathery in texture,
but free from the limestone pavement described above. The young (Figs.
128, 129) are very common, swimming freely about, and more readily
found than the adult; they are of a bright vermilion color, but the
tentacles hardly branch at that age, nor is the limestone pavement
formed, which gives such a peculiar aspect to the full-grown animal.
The young Cuvieria, somewhat older than that represented in Fig. 129,
are found in plenty under stones at low-water mark, just after they
have given up their nomadic habits, and when the limestone pavement
begins to be developed.
[Illustration: Fig. 128. Young Cuvieria, much enlarged; _l_ body,
_g_ tentacles.]
[Illustration: Fig. 129. Somewhat older Cuvieria; _l_ body, _g_
tentacle round mouth, _g'_ testaete of sole, _b_ madreporic
tentacle.]
_Pentacta_. (_Pentacta frondosa_ JÄG.)
The highest of our Holothurians in structure, is the Pentacta. (Fig.
130) It is very rare on our beaches, though occasionally found under
stones at low-water mark; farther north, in Maine, and at Grand Manan,
it is very common, covering all the rocks near low-water mark. It is a
chocolate brown in color, and measures, when fully expanded, some
fifteen to eighteen inches in length. Unlike the Cuvieria, the
ambulacral suckers are evenly distributed and almost equally developed
on all the tubes; between the five rows of ambulacral suckers are
scattered irregularly certain appendages resembling suckers, but found
on examination not to be true locomotive suckers, and called on that
account false ambulacra. These are the organs corresponding to the
warts around the mouth of the Synapta. Although the ambulacral suckers
are, as we have said, equally developed on all the tubes, yet the
Pentacta does not use them indiscriminately as locomotive organs. In
Pentacta, as well as in all Holothurians, whether provided with
ambulacral suckers, or, like the Synapta and Caudina, deprived of
them, the odd ambulacrum, viz. the one placed opposite the madreporic
body, is always used to creep upon, and forms the under surface of the
animal.
[Illustration: Fig. 130. Pentacta frondosa; expanded about one
third the natural size.]
The correspondence between the different phases of growth in the young
Pentacta, and the adult forms of the orders described above, the
Synapta, Caudina, Cuvieria, and Pentacta itself, is a striking
instance of the way in which embryonic forms illustrate the relative
standing of adult animals. In the earlier stages of its development,
the ambulacral tubes alone are developed in the Pentacta; in this
condition it recalls the lower orders of Holothurians, as the Synapta
and Caudina; then a sole is formed by the greater development of three
of the ambulacra, and in this state it reminds us of the next in
order, the Cuvieria, while it is only in assuming its adult form that
the Pentacta develops its other ambulacra, with their many suckers.
The Pentacta resembles the Trepang, so highly valued by the Chinese as
an article of food, and forms a not unsavory dish, having somewhat the
flavor of lobster.
* * * * *
ECHINOIDS.
_Sea-urchin_. (_Toxopneustes drobachiensis_ AG.)
[Illustration: Fig. 131. Toxopneustes from above, with all the
appendages expanded; natural size.]
Sea-urchins (Fig. 131) are found in rocky pools, hidden away usually
in cracks and holes. They like to shelter themselves in secluded
nooks, and, not satisfied even with the privacy of such a retreat,
they cover themselves with sea-weed, drawing it down with their
tentacles, and packing it snugly above them, as if to avoid
observation. This habit makes them difficult to find, and it is only
by parting the sea-weed, and prying into the most retired corners in
such a pool, that one detects them. Their motions are slow, and they
are less active than either the Star-fish or the Ophiuran, to both of
which they are so closely allied.
Let us look at one first, as seen from above, with all its various
organs fully extended. (Fig. 131.) The surface of the animal is
divided by ten zones, like ribs on a melon, only that these zones
differ in size, five broad zones alternating with five narrower ones.
The broad zones, representing the interambulacral system, are composed
of large plates, supporting a number of hard projecting spines, while
the narrow zones, forming the ambulacral system, are pierced with
small holes, arranged in regular rows, (Fig. 132,) through which
extend the tentacles terminating with little cups or suckers. These
zones converge towards the summit of the animal, meeting in the small
area which here represents the dorsal system; this area is filled by
ten plates, five larger ones at the extremity of the interambulacral
zones, and five smaller ones at the extremity of the ambulacral zones.
(Fig. 132.) In the five larger plates are the ovarian openings, so
called because each one is pierced by a small hole through which the
eggs are passed out, while in the five smaller plates are the
eye-specks. The ovaries themselves consist of long pouches or sacs,
carried along the inner side of each ambulacrum; one of these ovarian
plates is larger than the others, and forms the madreporic body, being
pierced with many minute holes; here, as in the Star-fish, it is
placed between two of the ambulacral rows, and opposite the fifth or
odd one. Looked at from the under or the oral side, as seen in Fig.
134, the animal presents the mouth, a circular aperture furnished with
five teeth in its centre; these five teeth opening into a complicated
intestine to be presently described. From the mouth, the ten zones
diverge, curving upward to meet in the dorsal area on the summit of
the body. (Fig. 133.)
[Illustration: Fig. 132. Portion of shell of Fig. 131, with spines
rubbed off. (_Agassiz_.)]
[Illustration: Fig. 133. Sea-urchin shell with all the spines
removed. (_Agassiz_.)]
Let us now examine the appearance and functions of the various
appendages on the surface. The tentacles have a variety of functions
to perform; they are the locomotive appendages, and for this reason,
as we have seen, the zones along which they are placed are called the
ambulacra, while the intervening spaces, or the broad zones, are
called the interambulacra. It should not be supposed, however, that
the locomotive appendages are the only ones to be found on the
ambulacra, for spines occur on the narrow as well as on the broad
zones, though the larger and more prominent ones are always placed on
the latter. The tentacles are also subservient to circulation, for the
water which is taken in at the madreporic body passes into all the
tentacles, sometimes called on that account water-tubes. Beside these
offices the tentacles are constantly busy catching any small prey, and
conveying it to the mouth, or securing the bits of sea-weed with
which, as has been said, these animals conceal themselves from
observation. It is curious to see their fine transparent feelers,
fastening themselves by means of the terminal suckers on such a
floating piece of sea-weed, drawing it gently down and packing it
delicately over the surface of the body. As locomotive appendages, the
tentacles are chiefly serviceable on the lower or oral side of the
animal, which always moves with the mouth downward. About this portion
of the body the tentacles are numerous (Fig. 134) and large, and when
the animal advances it stretches them in a given direction, fastens
them by means of the suckers on some surface, be it of rock, or shell,
or the side of the glass jar in which they are kept, and being thus
anchored it drags itself forward. The tentacles are of a violet hue,
though when stretched to their greatest length they lose their color,
and become almost white and transparent; but in their ordinary
condition the color is quite decided, and the rows along which they
occur make as many violet lines upon the surface of the body.
[Illustration: Fig. 134. Sea-urchin seen from the mouth side.
(_Agassiz_.)]
[Illustration: Fig. 135. Magnified spine.]
Almost the sole function of the spines seems to be that of protecting
the animal, and enabling it to resist the attacks of its enemies, the
force of the waves, or any sudden violent contact with the rocks. The
spines, when magnified, are seen to be finely ribbed for nearly the
whole length (Fig. 135), the bare basal knob serving as the point of
attachment for the powerful muscles, which move these spines on a
regular ball-and-socket joint, the ball surmounting the tubercles
(seen in Fig. 132), which fit exactly in a socket at the base of the
spine. In a transverse section of a spine (Fig. 136), we see that the
ribs visible on the outside are delicate columns placed closely side
by side, and connected by transverse rods forming an exceedingly
delicate pattern. Beside the tentacles and the spines, they have other
external appendages, of which the function long remained a mystery,
and is yet but partially explained; these are the so-called
pedicellariæ; they consist of a stem (_s_, Fig. 137), which becomes
swollen (_p_, Fig. 137) into a thimble-shaped knob at the end (_t_,
Fig. 137); this knob may seem solid and compact at first sight, but it
is split into three wedges, which can be opened and shut at will. When
open, these pedicellariæ may best be compared to a three-pronged fork,
except that the prongs are arranged concentrically instead of on one
plane, and, when closed, they fit into one another as neatly as the
pieces of a puzzle.
[Illustration: Fig. 136. Transverse section of spine; magnified.]
[Illustration: Fig. 137. Pedicellaria of Sea-urchin; _s_ stem, _p_
base of fork, _t_ fork.]
If we watch the Sea-urchin after he has been feeding, we shall learn,
at least, one of the offices which this singular organ performs in the
general economy of the animal. That part of his food which he ejects
passes out at an opening on the summit of the body, in the small area
where all the zones converge. The rejected particle is received on one
of these little forks, which closes upon it like a forceps, and it is
passed on from one to the other, down the side of the body, till it is
dropped off into the water. Nothing is more curious and entertaining
than to watch the neatness and accuracy with which this process is
performed. One may see the rejected bits of food passing rapidly along
the lines upon which these pedicellariæ occur in greatest number, as
if they were so many little roads for the conveying away of the refuse
matters; nor do the forks cease from their labor till the surface of
the animal is completely clean, and free from any foreign substance.
Were it not for this apparatus the food thus rejected would be
entangled among the tentacles and spines, and be stranded there till
the motion of the water washed it away. These curious little organs
may have some other office than this very laudable and useful one of
scavenger, and this seems the more probable because they occur over
the whole surface of the body, while they seem to pass the excrements
only along certain given lines. They are especially numerous about the
mouth, where they certainly cannot have this function; we shall see
also that they bear an important part in the structure of the
Star-fish, where there are no such avenues on the upper surface, for
the passage of the refuse food, as occur on the Sea-urchin.
[Illustration: Fig. 138. Teeth of Sea-urchin, so-called Lantern of
Aristotle.]
On opening a Sea-urchin, we find that the teeth (Fig. 138), which seem
at first sight only like five little conical wedges around the mouth
(Fig. 134), are connected with a complicated intestine, which extends
spirally from the lower to the upper floor of the body, festooning
itself from one ambulacral zone to the next, till it reaches the
summit, where it opens. This intestine leads into the centre of the
teeth, the jaws themselves, which sustain the teeth, being made up of
a number of pieces, and moved by a complicated system of muscular
bands. When the intestine is distended with food, it fills the greater
part of the inner cavity; the remaining space is occupied in the
breeding season by the genital organs. In a section of the Sea-urchin,
one may also trace the tube by which the supply of water, first
filtered through the madreporic body, is conveyed to the ambulacra; it
extends from the summit of the body to the circular tube surrounding
the mouth.
_Echinarachnius_. (_Echinarachnius parma_ GRAY.)
[Illustration: Fig. 139. Echinarachnius, seen from above, with the
spines on part of the shell; _a_ ambulacral zone, _i_
interambulacral zone.]
Beside the Toxopneustes (Fig. 131) described above, we have another
Sea-urchin very common along our shores. Among children who live near
sandy beaches, they are well known as "sand-cakes" (Fig. 139), and
indeed they are so flat and round, that, when dried and deprived of
their bristles, they look not unlike a cake with a star-shaped figure
on its surface. (Fig. 139.) When first taken from the water they are
of a dark reddish brown color, and covered with small silky bristles.
The disk is so flat, being but very slightly convex on the upper side,
that one would certainly not associate it at first sight with the
common spherical Sea-urchin or Sea-egg, as the Toxopneustes is
sometimes called. But upon closer examination the delicate ambulacral
tubes or suckers may be seen projecting from along the line of the
ambulacra, as in the spherical Sea-urchin; and though these ambulacra
become expanded near the summit into gill-like appendages, forming a
sort of rosette in the centre of the disk, they are, nevertheless, the
same organs, only somewhat more complicated. When such a disk is dried
in the sun, and the bristles entirely removed, the lines of suture of
the plates composing it, and corresponding exactly to those of the
spherical Sea-urchin, may very readily be seen. (_a_ and _i_, Fig.
139.)
[Illustration: Fig. 140. Transverse section of Echinarachnius; _o_
mouth, _e_ _e_ ambulacra, _c_ _m_ ambulacral ramifications, _w_
_w_ interambulacra. (_Agassiz_.)]
This flat Sea-urchin or Echinarachnius, as it is called, belongs to a
group of Sea-urchins known as Clypeastroids (shield-like Sea-urchins).
In a section (Fig. 140) exposing the internal structure, one cannot
but be reminded by its general aspect of an Aurelia. Could one
solidify an Aurelia it would present much the same appearance; another
evidence that all the Radiates are built on one plan, their
differences being only so many modes of expressing the same structural
idea. The teeth or jaws in this flat Sea-urchin are not so complicated
as in the Toxopneustes, being simply flat pieces, arranged around the
mouth (_o_, Fig. 140), without the apparatus of muscular bands by
means of which the teeth are moved in the other genus. It is a curious
fact, considered in relation to the general radiate structure of these
animals, that the teeth, instead of moving up and down like the jaws
in Vertebrates, or from right to left like those of Articulates, move
concentrically, all converging towards the centre.
* * * * *
STAR-FISHES.
_Star-fish_. (_Astracanthion berylinus_ AG.)
Although there is the closest homology of parts between the Star-fish
and the Sea-urchin, the arrangement of these parts, and the external
appearance of the animals, as a whole, are entirely different. The
Star-fish has zones corresponding exactly to those of the Sea-urchin,
but instead of being drawn together, and united at the summit of the
animal, so as to form a spherical outline, they are spread out on one
level in the shape of a star. This change in the general arrangement
brings the eye-specks to the extremities of the arms, and places the
ovarian openings in the angles between the arms. The madreporic body
is situated on the upper surface of the disk (Fig. 142), at the angle
between two of the arms, and consequently between two of the
ambulacra, and opposite the odd one. The tube into which it opens,
runs vertically from the upper floor of the disk to the lower, where
it connects with the circular tube around the mouth, and thus
communicates with all the ambulacral rows. The ambulacral zones which,
in the Star-fish, have the shape of a furrow, run along the lower side
of each ray (Fig. 141); the interambulacral zones are divided, their
plates being arranged in rows along either side of the ambulacral
furrows. The ambulacral furrow, like the ambulacral zone in the
Sea-urchin, is pierced with numerous holes, alternating with each
other in a kind of zigzag arrangement, one hole a little in advance,
the next a little farther back, and so on, and through these holes
pass the tentacles, terminating in suckers, as in the Sea-urchins, and
serving as in them for locomotive organs. The most prominent and
strongest spines are arranged upon the large interambulacral plates on
both sides of the ambulacral furrows; but the upper surface of the
animal is also completely studded with smaller spines, scattered at
various distances, apparently without any regular arrangement.
(Fig. 142.)
[Illustration: Fig. 141. Star-fish ray, seen from mouth side.
(_Agassiz_.)]
[Illustration: Fig. 142. Star-fish; natural size, seen from
above.]
The position of the pedicellariæ is quite different from that which
they occupy in the Sea-urchin, where they are scattered singly between
the spines and tentacles, though more regularly and closely grouped
along the lines upon which the refuse food is moved off. In the
Star-fish, on the contrary, these singular organs seem to be grouped
for some special purpose around the spines, on the upper surface of
the body. Every such spine swells near its point of attachment, thus
forming a spreading base (Fig. 143), around which the pedicellariæ are
arranged in a close wreath, in the centre of which the summit of the
spine projects; they differ also from those of the Sea-urchin in
having two prongs instead of three. Other pedicellariæ are scattered
independently over the surface of the animal, but they are smaller
than those forming the clusters and connected with the spines. The
function of these organs in the Star-fish remains unexplained; the
opening on the upper surface, through which the refuse food is thrown
out, is in such a position that they evidently do not serve here the
same purpose which renders them so useful to the Sea-urchin.
Occasionally they may be seen to catch small prey with these forks,
little Crustacea, for instance; but this is probably not their only
office. The Star-fish has a fourth set of external appendages in the
shape of little water-tubes. (Seen in Fig. 143.) The upper surface of
the back consists of a strong limestone network (Fig. 144), and
certain openings in this network are covered with a thin membrane
through which these water-tubes project. It is supposed that water may
be introduced into the body through these tubes; but while there can
be no doubt that they are constantly filled with water, and are
therefore directly connected with the circulation through the
madreporic body (Fig. 145), no external opening has as yet been
detected in them. The fact, however, that when these animals are taken
out of their native element, the water pours out of them all over the
surface of the back, so that they at once collapse and lose entirely
their fulness of outline, seems to show that water does issue from
those tubes. The ends of the arms are always slightly turned up, and
at the summit of each is a red eye-speck. The tentacles about the eye
become very delicate and are destitute of suckers.
[Illustration: Fig. 143. Single spine of Star-fish, with
surrounding appendages; magnified.]
[Illustration: Fig. 144. Limestone network of back of Star-fish.]
[Illustration: Fig. 145. Madreporic body of Star-fish; magnified.]
These animals have singular mode of eating; they place themselves over
whatever they mean to feed upon, as a cockle-shell for instance, the
back gradually rising as they arch themselves above it; they then turn
the digestive sac or stomach inside out, so as to enclose their prey
completely, and proceed leisurely to suck out the animal from its
shell. Cutting open any one of the arms we may see the yellow folds of
the stomach pouches which extend into each ray; within the arms,
extending along either side of the upper surface, are also seen the
ovaries, like clusters of small yellow berries. Immediately below
these, along the centre of the lower floor of each ray, runs the ridge
formed by the ambulacral furrow, and upon either side of this ridge
are placed the vesicles, by means of which the tentacles may be filled
and emptied at the will of the animal; the rest of the cavity of the
ray is filled by the liver. The mouth, which is surrounded by a
circular tube, is not furnished with teeth, as in the Sea-urchin; but
the end of each ambulacral ridge is hard, thus serving the purpose of
teeth.
_Cribrella_. (_Cribrella oculata_ FORBES.)
Our coast, as we have said, is not rich in the variety of Star-fishes.
We have two large species, one of a dark-brown color (Fig. 132), the
_Astracanthion berylinus_, and the other, the _A. pallidus_, of a
pinkish tint; then there is the small Cribrella, inferior in
structural rank to the two above mentioned. (Fig. 146.) This pretty
little Star-fish presents the greatest variety of colors; some are
dyed in Tyrian purple, others have a paler shade of the same hue, some
are vermilion, others a bright orange or yellow. A glass dish filled
with Cribrellæ might vie with a tulip-bed in gayety and vividness of
tints.
[Illustration: Fig. 146. Cribrella from above; natural size.]
The disk of the Cribrella is smooth, instead of being covered, like
the larger Star-fishes, with a variety of prominent appendages. The
spines are exceedingly short, crowded like little warts over the
surface. It is an interesting fact, illustrating again the
correspondence between the adult forms of the lower orders and the
phases of growth in the higher ones, that these spines have an
embryonic character. One would naturally expect to find that these
small spines of the adult Cribrella would differ from those of the
other full-grown Star-fishes chiefly in size, that they would be a
somewhat modified pattern of the same thing on a smaller scale; but
when examined under the microscope, they resemble the spines of the
higher orders in their embryonic condition; it is not, in fact, a
difference in size merely, but a difference in degree of development.
The Cribrella moves usually with two of the arms turned backward, and
the three others advanced together, the two posterior ones being
sometimes brought so close to each other as to touch for their whole
length.
_Hippasteria_. (_Hippasteria phrygiana_ AG.)
Beside these Star-fishes we have the pentagonal Hippasteria
(_Hippasteria phrygiana_ AG.), like a red star with rounded points,
found chiefly in deep water, though it is occasionally thrown up on
the beaches. It has but two rows of large tentacles, terminating in a
powerful sucking disk. The pedicellariæ on this Star-fish resemble
large two-pronged clasps, arranged principally along the lower side.
The pentagonal Star-fishes of our coast are in striking contrast to
the long-armed species we have just described; they are edged with
rows of large smooth plates, and do not possess the many prominent
spines so characteristic of the ordinary Star-fishes.
_Ctenodiscus_. (_Ctenodiscus crispatus_ D. & K.)
The Ctenodiscus (_Ctenodiscus crispatus_ D. & K., Fig. 147), an
inhabitant of more northern waters, but seeming also to be at home
here occasionally, is another pentagonal Star-fish. It lives in deep
water, and frequents muddy bottoms. The peculiar structure of their
ambulacra has probably some reference to this mode of living, for they
are entirely wanting in the sucking disks so characteristic of the
other members of this class, and their tentacles are pointed, as if to
enable them to work their way through the mud in which they make their
home. The pointed tentacles of this genus are characteristic of a
large group of Star-fishes, and it is an important fact, as showing
their lower standing, that this feature, as well as the pentagonal
outline, obtains in the earlier stages of growth of our more common
Star-fishes, while in their adult condition they assume the deeply
indented star-shaped outline, and have suckers at the extremities of
the tentacles.
[Illustration: Fig. 147. Ctenodiscus, seen from above; natural
size.]
_Solaster_. (_Solaster endeca_ FORBES.)
We find also among Star-fishes the same tendency to multiplication of
parts so common among the Polyps and Acalephs. Our Solaster (_Solaster
endeca_ Forbes), for instance, has no less than twelve arms; it
inhabits more northern latitudes, though sometimes found in our Bay;
on the coast of Maine it is quite common, and occurs in company with
another many-rayed species, the _Crossaster papposa_ M. & T. The color
of both of these Star-fishes is exceedingly varied; we find in the
Solaster as many different hues as in the Cribrella, which it
resembles in the structure of its spines, while in the Crossaster
bands of different tints of red and purple are arranged
concentrically, and the whole surface of the back is spotted with
brilliantly-tinged tiny wreaths of water-tubes, crowded round the base
of the different spines, which are somewhat similar to those of the
Astracanthion.
OPHIURANS.
_Ophiopholis_. (_Ophiopholis bellis_ LYM.)
[Illustration: Fig. 148. Ophiopholis, from above; natural size.]
There are but two species of the ordinary forms of Ophiurans in
Massachusetts Bay; the white Amphiura (_Amphiura squamata_ Sars), with
long slender arms, and the spotted Ophiopholis (Fig. 148), with
shorter and stouter arms, and in which the disk is less compact than
in the Amphiura, and not so perfectly circular. All Ophiurans are
difficult to find, from their exceeding shyness; they hide themselves
in the darkest crevices, and though no eye-specks have yet been
detected in them, they must have some quick perception of coming
danger, for at the gentlest approach they instantly draw away and
shelter themselves in their snug retreats. [Illustration: Fig. 149.
One arm of Fig. 148; from the mouth side.]
[Illustration: Fig. 150. Ambulacral tentacle of Ophiopholis;
magnified.]
They differ from the Star-fishes in having the disk entirely distinct
from the arms; that is, the arms, instead of merging gradually into
the disk, start at once from its margin. They have no interambulacral
spaces or plates; but the whole upper surface is formed of large hard
plates, which extend from the back over the sides of the arms to their
lower surface, where they form a straight ridge along the centre.
(Fig. 149.) The sides of these plates are pierced with holes, through
which the tentacles pass; these have not, like those of the
Star-fishes and Sea-urchins, a sucker at the extremity, but are
covered with little warts or tubercles (Fig. 150); they are their
locomotive appendages, and their way of moving is curious; they first
extend one of the arms in the direction in which they mean to move,
then bring forward two others to meet them, three arms being thus
usually in advance, and then they drag the rest of the body on. They
move with much more rapidity, and seem more active, than the
Star-fishes; probably owing to the greater independence of the arms
from the disk. The spines project along the margin of the arms, and
not over the whole surface, the back of the arms being perfectly free
from any appendages, and presenting only the surface of the plates.
The madreporic body is formed by a plate on the lower side of the
disk, in a position corresponding to that which it occupies in the
young Star-fish; this plate is one of the large circular shields
occupying the interambulacral spaces around the mouth. (Fig. 149.) On
each side of the arms, where they join the disk, are slits opening
into the ovarian pouches. They have no teeth; but the hard ridge at
the oral end of the ambulacra, extending toward the mouth in
Star-fish, is still more distinct and sharper in the Ophiurans,
approaching more nearly the character of teeth.
_Astrophyton_. (_Astrophyton Agassizii_ STIMP.)
A singular species of Ophiuran, known among fishermen as the
"Basket-fish," (Fig. 151,) is to be found in Massachusetts Bay. Its
arms are very long in comparison to the size of the disk, and divide
into a vast number of branches. In moving, the animal lifts itself on
the extreme end of these branches, standing as it were on tiptoe (Fig.
151), so that the ramifications of the arms form a kind of
trellis-work all around it, reaching to the ground, while the disk
forms a roof. In this living house with latticed walls small fishes
and other animals are occasionally seen to take shelter; but woe to
the little shrimp or fish who seeks a refuge there, if he be of such a
size as to offer his host a tempting mouthful; he will fare as did the
fly who accepted the invitation of the spider. These animals are
exceedingly voracious, and sometimes, in their greediness for food,
entangle themselves in fishing lines or nets. When disturbed, they
coil their arms closely around the mouth, assuming at such times a
kind of basket-shape, from which they derive their name.
This Basket-fish is honorably connected with our early colonial
history, being thought worthy, by no less a personage than John
Winthrop, Governor of Connecticut, who, as he says, "had never seen
the like," to be sent with "other natural curiosities of these parts"
to the Royal Society of London, in 1670. He accompanies the specimen
with a minute description, omitting "other particulars, that we may
reflect a little upon this elaborate piece of nature." His account is
as graphic as it is accurate, and we can hardly give a better idea of
the animal than by extracting some portions of it. "This Fish," he
says, "spreads itself from a Pentagonal Root, which incompasseth the
Mouth (being in the middle), into 5 main Limbs or branches, each of
which, just at issuing out from the Body, subdivides itself into two,
and each of these 10 branches do again divide into two parts, making
20 lesser branches; each of which again divide into two smaller
branches, making in all 40. These again into 80, and these into 160;
and these into 320; these into 640; into 1280; into 2560; into 5120;
into 10,240; into 20,480; into 40,960; into 81,920; beyond which the
further expanding of the Fish could not be certainly trac'd";--a
statement which we readily believe, wondering only at the patience
which followed this labyrinth so far.
[Illustration: Fig. 151. Astrophyton, Basket-fish; in a natural
attitude.]
In a later letter, after having had an interview with the fisherman
who caught the specimen, and, as he says, "asked all the questions I
could think needful concerning it," the Governor proceeds to tell us
that it was caught "not far from the Shoals of Nantucket (which is an
Island upon the Coast of New England)," and that when "first pull'd
out of the water it was like a basket, and had gathered itself round
like a Wicker-basket, having taken fast hold upon that bait on the
hook which he" (the fisherman) "had sunk down to the bottom to catch
other Fish, and having held that within the surrounding brachia would
not let it go, though drawn up into the Vessel; until, by lying a
while on the Deck, it felt the want of its natural Element; and then
voluntarily it extended itself into the flat round form, in which it
appear'd when present'd to your view." The Governor goes on to reflect
in a philosophical vein upon the purpose involved in all this
complicated machinery. "The only use," he says, "that could be
discerned of all that curious composure wherewith nature had adorned
it seems to be to make it as a purse-net to catch some other fish, or
any other thing fit for its food, and as a basket of store to keep
some of it for future supply, or as a receptacle to preserve and
defend the young ones of the same kind from fish of prey; if not to
feed on them also (which appears probable the one or the other), for
that sometimes there were found pieces of Mackerel within that
concave. And he, the Fisherman, told me that once he caught one, which
had within the hollow of its embracements a very small fish of the
same kind, together with some piece or pieces of another fish, which
was judged to be of a Mackerel. And that small one ('tis like) was
kept either for its preservation or for food to the greater; but,
being alive, it seems most likely it was there lodged for safety,
except it were accidentally drawn within the net, together with that
piece of fish upon which it might be then feeding." The account
concludes by saying, "This Fisherman could not tell me of any name it
hath, and 'tis in all likelihood yet nameless, being not commonly
known as other Fish are. But until a fitter _English_ name be found
for it, why may it not be called (in regard of what hath been before
mentioned of it) a _Basket-Fish_, or a _Net-Fish_, or a
_Purs-net-Fish_?" And so it remains to this day as the Governor of
Connecticut first christened it, the Basket-fish.
* * * * *
CRINOIDS.
[Illustration: Fig. 152. Fossil Pentacrinus.]
The Crinoids are very scantily represented in the present creation.
They had their day in the earlier geological epochs, when for some
time they remained the sole representatives of their class, and were
then so numerous that the class of Echinoderms, with only one order,
seemed as full and various as it now does with five. The different
forms they assumed in the successive geological periods are
particularly instructive; these older Crinoids combined characters
which foreshadowed the advent of the Ophiurans, the true Star-fishes,
and the Sea-urchins; and so prominently were their prophetic
characters developed, that many of them are readily mistaken for
Star-fishes or Sea-urchins.
In later times the group of Crinoids has been gradually dwindling in
number and variety. Its present representatives are the Pentacrini of
Porto Rico and the coast of Portugal, the lovely little Rhizocrinus of
the Atlantic, dredged first by the younger Sars on the coast of
Norway, attached throughout life to a stem, and the Comatula, which
has a stem only in the early stages of its growth, but is free when
adult. The Pentacrinus bears the closer relation to the more ancient
Crinoids (Fig. 152), which were always supported on a stem, while it
is only in more recent periods that we find the free Crinoids,
corresponding to the Comatula.
_Comatula_. (_Alecto meridionalis_ AG.)
One large species of Comatula (_Alecto Eschrichtii_ M. & T.) is known
on our coast, off the shores of Greenland, where it has been dredged
at a depth of about one hundred and fifty fathoms, and young specimens
of the same species have been found as far south as Eastport, Maine.
The species selected for representation here, however, (Fig. 153,) is
one quite abundant along the shores of South Carolina. It is
introduced instead of the northern one, because the latter is so rare
that it is not likely to fall into the hands of our readers. The
annexed drawing (Fig. 154, magnified from Fig. 153) represents a group
of the young of the Charleston Comatula, still attached to the parent
body by their stems, and in various stages of development. At first
sight, the Comatula, or, as it is sometimes called, the feather-star,
resembles an Ophiuran; but on a closer examination we find that the
arms are made up of short joints; and along the sides of the arms,
attached to each joint, are appendages resembling somewhat the beards
of a feather, and giving to each ray the appearance of a plume; hence
the name of feather-star. On one side the arms are covered with a
tough skin, through which project the ambulacræ, and on the same side
of the disk are situated the mouth and the anus; the latter projects
in a trumpet-shaped proboscis. On the opposite side of the disk the
Comatula is covered with plates, arranged regularly around a central
plate, which is itself covered with long cirri.
[Illustration: Fig. 153. Comatula (Living Crinoid) seen from the
back; a group of young Comatulæ attached to parent.]
[Illustration: Fig. 154. Magnified view of the group of young
Comatulæ of Fig. 153.]
We are indebted to Thompson for the explanation of the true relations
of the young Comatula to the present Pentacrinus and the fossil
Crinoids. Supposing these young to be full-grown animals, he at first
described them as living representatives of the genus Pentacrinus; it
was only after he had watched their development, and ascertained by
actual observation that they dropped from their stem, to lead an
independent life as free Comatulæ, that he fully understood their true
connection with the past history of their kind, as well as with their
contemporaries. In Fig. 153, a faint star-like dot (_y_) may be seen
attached to the side of the disk by a slight line. In Fig. 154, we
have that minute dot as it appears under the microscope, magnified
many diameters; when it is seen to be a cirrus of a Comatula, with
three small Pentacrinus-like animals growing upon it, in different
stages of development. In the upper one, the branching arms and the
disk, with its many plates, are already formed; and though in the
figure the rays are folded together, they are free, and can be opened
at will. In the larger of the two lower buds, the plates of the disk
are less perfect, and the arms are straight and simple, without any
ramifications, though they are free and movable, whereas, in the
smaller one, they are folded within the closed bud.
* * * * *
EMBRYOLOGY OF ECHINODERMS.
All Radiates have a special mode of development, as distinct for each
class as is their adult condition, and in none are the stages of
growth more characteristic than in the Echinoderms. In the Polyps, the
division of the body into chambers, so marked a feature of their
ultimate structure, takes place early; in the Acalephs, the tubes
which traverse the body are hollowed out of its mass in the first
stages of the embryonic growth, and we shall see that in the
Echinoderms also, the distinctive feature of their structure, viz. the
enclosing of the organs by separate walls, early manifests itself.
This peculiarity gives to the internal structure of these animals so
individual a character, that some naturalists, overlooking the law of
radiation, as prevalent in them as in any members of this division,
have been inclined to separate them, as a primary division of the
animal kingdom, from the Polyps and Acalephs, in both of which the
body-wall furnishes the walls of the different internal cavities,
either by folding inwardly in such a manner as to enclose them, as in
the Polyps, or by the cavities themselves being hollowed out of the
general mass, as in the Acalephs.
_Star-fish_. (_Astracanthion_.)
[Illustration: Fig. 155. Egg of Star-fish.]
[Illustration: Fig. 156. Egg of Star-fish in which the yolk has
been divided into two segments.]
[Illustration: Fig. 157. Egg in which there are eight segments of
the yolk.]
The egg of the Star-fish, when first formed, is a transparent,
spherical body, enclosing the germinative vesicle and dot. (See Fig.
155.) As soon as these disappear, the segmentation of the yolk begins;
it divides first into two portions (see Fig. 156), then into four,
then into eight, and so on; but when there are no more than eight
bodies of segmentation (see Fig. 157), they already show a disposition
to arrange themselves in a hollow sphere, enclosing a space within,
and by the time the segmentation is completed, they form a continuous
spherical shell. At this time the egg, or, as we will henceforth call
it, the embryo, escapes and swims freely about. (See Fig. 158.) The
wall next begins to thin out on one side, while on the opposite side,
which by comparison becomes somewhat bulging, a depression is formed
(_m_ _a_, Fig. 159), gradually elongating into a loop hanging down
within the little animal, and forming a digestive cavity. (_d_, Fig.
160.) At this stage it much resembles a young Actinia. The loop
spreads somewhat at its upper extremity, and at its lower end is an
opening, which at this period of the animal's life serves a double
purpose, that of mouth and anus also, for at this opening it both
takes in and rejects its food. We shall see that before long a true
mouth is formed, after which this first aperture takes its place
opposite the mouth, retaining only the function of the anus. Presently
from the upper bulging extremity of the digestive cavity, two lappets,
or little pouches, project (_w_ _w'_ Fig. 161); they shortly become
completely separated from it, and form two distinct hollow cavities
(_w_ _w'_, Fig. 162). Here begins the true history of the young
Star-fish, for these two cavities will develop into two water-tubes,
on one of which the back of the Star-fish, that is, its upper surface,
covered with spines, will be developed, while on the other, the lower
surface, with the suckers and tentacles, will arise. At a very early
stage one of these water-tubes (_w'_, Fig. 163) connects with a
smaller tube opening outwards, which is hereafter to be the madreporic
body (_b_, Fig. 163). Almost until the end of its growth, these two
surfaces, as we shall see, remain separate, and form an open angle
with one another; it is only toward the end of the development that
they unite, enclosing between them the internal organs, which have
been built up in the mean while.
[Illustration: Fig. 158. Larva just hatched from egg; a thickened
pole.]
[Illustration: Fig. 159. Larva somewhat older than Fig. 158; _m_
_a_ depression at thickened pole.]
[Illustration: Fig. 160. Larva where the depression has become a
digestive cavity _d_, opening at _a_.]
[Illustration: Fig. 161. Earlets, _w_ _w'_ (water-tubes),
developed at the extremity of the digestive cavity _d_; _m_
mouth.]
[Illustration: Fig. 162. More advanced larva; _a_ _d_ _c_
digestive system, _v_ vibratile chord, _m_ mouth.]
[Illustration: Fig. 163. Profile view of larva; _b_ madreporic
opening, _w'_ earlet, _a_ _d_ digestive system, _m_ mouth, _v_
_v'_ vibratile chord.]
[Illustration: Fig. 164. Larva showing mode of formation of mouth
_m_, by bending of digestive cavity _o_.]
At about the same time with the development of these two pouches, so
important in the animal's future history, the digestive cavity becomes
slightly curved, bending its upper end sideways till it meets the
outer wall, and forms a junction with it (_m_, Fig. 164). At this
point, when the juncture takes place, an aperture is presently formed,
which is the true mouth. The digestive sac, which has thus far served
as the only internal cavity, now contracts at certain distances, and
forms three distinct, though connected cavities, as in Fig. 163; viz.
the oesophagus leading directly from the mouth (_m_) to the second
cavity or stomach (_d_), which opens in its turn into the third
cavity, the alimentary canal. Meanwhile the water-tubes have been
elongating till they now surround the digestive cavity, extending on
the other side of it beyond the mouth, where they unite, thus forming
a Y-shaped tube, narrowing at one extremity, and dividing into two
branches toward the other end. (Fig. 165.)
[Illustration: Fig. 165. Larva in which arms are developing,
lettering as before; _e'_ _e''_ _e'''_ _e^4_ _e^5_ _e^6_ arms, _o_
oesophagus.]
On the surface where the mouth is formed, and very near it on either
side, two small arcs arise, as _v_ in Fig. 162; these are cords
consisting entirely of vibratile cilia. They are the locomotive organs
of the young embryo, and they gradually extend until they respectively
enclose nearly the whole of the upper and lower half of the body,
forming two large shields or plastrons. (Figs. 165, 166.) The corners
of these shields project, slightly at first (Fig. 165), but elongating
more and more until a number of arms are formed, stretching in various
directions (Figs. 166, 167), and, by their constant upward and
downward play, moving the embryo about in the water.
[Illustration: Fig. 166. Adult larva, so-called Brachiolaria,
lettering as before; _r_ back of young Star-fish, _t_ tentacles of
young Star-fish, _f_ _f'_ brachiolar appendages.]
[Illustration: Fig. 167. Fig. 166 seen in profile, lettering as
before.]
[Illustration: Fig. 168. Star-fish which has just resorbed the
larva, seen from the back; _b_ madreporic opening.]
[Illustration: Fig. 169. Fig. 168, seen from the mouth side; _m_
mouth, _t_ tentacles.]
[Illustration: Fig. 170. Young Star-fish which has become
symmetrical, seen from the back; _t'_ odd tentacle.]
At this stage of the growth of the embryo, we have what seems quite a
complicated structure, and might be taken for a complete animal; this
is after all but the prelude to its true Star-fish existence. While
these various appendages of the embryo have been forming, changes of
another kind have taken place; on one of the two water-tubes above
mentioned (_w'_), at the end nearest the digestive cavity, a number of
lobes are formed (_t_, Fig. 166); this is the first appearance of the
tentacles. In the same region of the opposite water-tube (_w_) a
number of little limestone rods arise, which eventually unite to form
a continuous network; this is the beginning of the back of the
Star-fish (_r_, Fig. 166), from which the spines will presently
project. When this process is complete, the whole embryo, with the
exception of the part where the young Star-fish is placed, grows
opaque; it fades, as it were, begins to shrink and contract, and
presently drops to the bottom, where it attaches itself by means of
short arms (_f_ _f'_, Fig. 166), covered with warts, which act as
suckers, and are placed just above the mouth. As soon as the Star-fish
has thus secured itself, it begins to resorb the whole external
structure described above; the water-tubes, the plastrons, and the
complicated system of arms connected with them, disappear within the
little Star-fish; it swallows up, so to speak, the first stage of its
own existence; it devours its own larva, which now becomes part and
parcel of the new animal. Next the two surfaces, the back and lower
surface, on which the arms are now marked out, while the tentacles,
suckers, and spines have already assumed a certain prominence,
approach each other. At this time, however, the arms are not in one
plane; both the back and the lower surface are curved in a kind of
spiral; they begin to flatten; the arms spread out on one level,--and
now the two surfaces draw together, meeting at the circumference, and
enclosing between them the internal organs, which, as we have seen,
are already formed and surrounded by walls of their own, before the
two walls of the body, close thus over them. Fig. 168 represents the
upper surface of the Star-fish just before this junction takes place.
The complicated structure of the Brachiolaria, as the larva of the
Star-fish has been called, hitherto so essential to the life of the
animal, by which it has been supported, moved about in the water, and
provided with food during its immature condition, has made a final
contribution to its further development by the process of resorption
described above, and has wholly disappeared within the Star-fish. At
this stage the rays are only just marked out, as five lobes around the
margin; Fig. 169 represents the lower surface at the same moment, with
the open mouth (_m_), around which the tentacles (_t_) are just
beginning to appear; while Fig. 170 shows us the animal at a more
advanced stage, after the two surfaces have united. It has now
somewhat the outline of a Maltese cross, the five arms being more
distinctly marked out, while the tentacles have already attained a
considerable length (Fig. 171), and the dorsal plates have become
quite distinct. Fig. 172 represents the same animal, at the same age,
in profile. This period, in which we have compared the form of the
Star-fish to that of a Maltese cross, is one of long duration; two or
three years must elapse before the arms will elongate sufficiently to
give it a star-shaped form, and before the pedicellariæ make their
appearance, and it is only then that it can be at once recognized as
the young of our common Star-fish. Even then, after it has assumed its
ultimate outline, it lacks some features of the adult, having only two
rows of tentacles, whereas the full-grown Star-fish has four.
[Illustration: Fig. 171. Lower side of ray of young Star-fish; _m_
mouth, _b_ madreporic body, _e_ eye-speck.]
[Illustration: Fig. 172. Young Star-fish seen in profile; _t'_ odd
tentacle at extremity of arm.]
_Sea-urchins_.
[Illustration: Fig. 173, 174, 175. Young larvæ of Toxopneustes in
different stages of development; _e'_-_e^iv_ arms, _v-v''_
vibratile chord, _w_ _w'_ earlets (water-tubes), _a_ _o_ _d_ _c_
digestive system, _r'-r'''_ solid rods of arms, _m_ mouth, _b_
madreporic opening.]
This extraordinary process of development which we have analyzed thus
at length in the history of the Star-fish, but which is equally true
of all Echinoderms, has been hitherto described (so far as it was
known) under the name of the plutean stages of growth. In these early
stages the young, or the so called larvæ of Echinoderms, have received
the name of Pluteus on account of their ever-changing forms. Let us
look for a moment at the plutean stages of the Sea-urchin, as they
differ in some points from those of the Star-fish. In the Pluteus of
our common Sea-urchins (see Fig. 176), the arms are supported by a
framework of solid limestone rods, which do not exist in that of the
Star-fish, and which give to the larva of the Sea-urchin a remarkable
rigidity. They are formed very early, as may be seen in Fig. 173,
representing the little Sea-urchin before any arms are discernible,
though the limestone rods are quite distinct. Figs. 173, 174, 175, may
be compared with Figs. 160, 162, 165, of the young Star-fish, where it
will be seen that the general outline is very similar, though, on
account of the limestone rods, the Pluteus of the Sea-urchin seems
somewhat more complicated. In Fig. 176 the young Sea-urchin has so far
encroached upon the Pluteus that it forms the essential part of the
body, the arms and rods appearing as mere appendages. Fig. 177 shows
the same animal when we looked down upon it in its natural attitude;
the Sea-urchin is carried downward, and the arms stretch in every
direction around it. In Fig. 178 the Plutens is already in process of
absorption; in Fig. 179 it has wholly disappeared; in Figs. 180 and
181 we have different stages of the little Sea-urchin, with its spines
and suckers of a large size and in full activity. The appearance of
the Sea-urchin, as soon as this larva or Pluteus is completely
absorbed, is much more like that of the adult than is the Star-fish at
the same stages, in which, as we have seen, there is a transition
period of considerable duration.
[Illustration: Fig. 176. Adult larva of Toxopneustes, _f_
brachiolar appendages.]
[Illustration: Fig. 177. Fig. 176 seen endways.]
[Illustration: Fig. 178. The Sea-urchin resorbing the arms of the
larva.]
[Illustration: Fig. 179. Half a young Sea-urchin immediately after
resorption of the larva; _s''_ _s''_ spines, _t'_ _t'_ ambulacral
tentacles.]
[Illustration: Fig. 180. Young Sea-urchin older than Fig. 179; _t_
_t'_ tentacles, _s''_ _s'''_ spines.]
[Illustration: Fig. 181. Still older Sea-urchin; _t_ _t_
tentacles, _a_ anus, _p_ pedicellariæ; shell one sixteenth of an
inch in diameter.]
_Ophiurans_.
Fig. 183 represents an Ophiuran undergoing the same process of growth,
at a period when the larva is most fully developed, and before it
begins to fail. By the limestone rods which support the arms, the
Pluteus of the Ophiuran, here represented, resembles that of the
Sea-urchin more than that of the Star-Fish, while by the character of
the water-tubes and by its internal organization it is more closely
allied to the latter. It differs from both, however, in the immense
length of two of the arms; these arms being the last signs of its
plutean condition to disappear; when the young Ophiuran has absorbed
almost the whole Pluteus, it still goes wandering about with these two
immense appendages, which finally share the fate of all the rest. Fig.
182 represents an Ophiuran at the moment when the process of
resorption is nearly completed, though the arms of the Pluteus,
greatly diminished, are still to be seen protruding from the surface
of the animal.
[Illustration: Fig. 182. Ophiuran which has resorbed the whole
larva except the two long arms, _y_ _y'_ limestone rods of young
Ophiuran, _r_ middle of back, lettering as in Fig. 183.]
[Illustration: Fig. 183. Larva of Ophiuran; _e'-e^iv_ arms, _r'_
_r^iv_ solid rods, _v_ _v'_ vibratile chord, _w_ _w'_ water
system, _b_ madreporic body, _a_ _d_ digestive system.]
[Illustration: Fig. 184 Young Ophiuran which has resorbed the
whole larva; _r_ middle plate of back.]
[Illustration: Fig. 185 Cluster of eggs of Star-fishes placed over
the mouth of the parent.]
This mode of development, though common to all Echinoderms, appears
under very different conditions in some of them. There are certain
Star-fishes, Ophiurans, and Holothurians, passing through their
development under what is known as the sedentary process. The eggs are
not laid, as in the cases described above, but are carried in a sort
of pouch over the mouth of the parent animal, where they remain till
they attain a stage corresponding to that of Fig. 168 of the
Star-fish, and having much the same cross-shaped outline, when they
escape from the pouch (as the young Ophiopholis, Fig. 184), and swim
about for the first time as free animals. Fig. 185 represents a
cluster of young Star-fishes of the sedentary kind at about this
period. But while this mode of growth seems at first sight so
different, we shall find, if we look a little closer, that it is
essentially the same, and that, though the circumstances under which
the development takes place are changed, the process does not differ.
The little Star-fish or Ophiuran, in the pouch, becomes surrounded by
the same plutean structure as those which are laid in the egg; it is
only more contracted to suit the narrower space in which they have to
move; and the water-tubes on which the upper and lower surfaces of the
body arise, the shields, spreading out into arms at the corners,
exist, fully developed or rudimentary, in the one as much as in the
other, and when no longer necessary to its external existence they are
resorbed in the same way in both cases. This singular process of
development has no parallel in the animal kingdom, although the growth
of the young Echinoderm on the Brachiolaria may at first sight remind
us of the budding of the little Medusa on the Hydroid stock, or even
of the passage of the insect larva into the chrysalis. But in both
these instances, the different phases of the development are entirely
distinct; the Hydroid stock is permanent, continuing to live and grow
and perform its share in the cycle of existence to which it belongs,
after the Medusa has parted from it to lead a separate life, or if the
latter remains attached to the parent stock, after it has entered upon
its own proper functions. The life of the caterpillar, chrysalis and
butterfly, is also distinct and definitely marked; the moment when the
animal passes from one into the other cannot be mistaken, although the
different phases are carried on successively and not simultaneously,
as in the case of the Acalephs. But in the Echinoderms, on the
contrary, though the aspect of the Brachiolaria, or plutean stage, is
so different from that of the adult form, that no one would suppose
them to belong to the same animal, yet these two stages of growth pass
so gradually into one another, that one cannot say when the life of
the larva ceases, and that of the Echinoderm begins.
The bearing of embryology upon classification is becoming every day
more important, rendering the processes of development among animals
one of the most interesting and instructive studies to which the
naturalist can devote himself, in the present state of his science.
The accuracy of this test, not only as explaining the relations
between animals now living, but as giving the clew to their connection
with those of past times, cannot but astonish any one who makes it the
basis of his investigations. The comparison of embryo forms with
fossil types is of course difficult, and must in many instances be
incomplete, for while, in the one case, death and decay have often
half destroyed the specimen, in the other, life has scarcely stamped
itself in legible characters on the new being. Yet, whenever such
comparisons have been successfully carried out, the result is always
the same; the present representatives of the fossil types recall in
their embryonic condition the ancient forms, and often explain their
true position in the animal kingdom. One of the most remarkable
examples of this in the type we are now considering, is that of the
Comatula already mentioned. Its condition in the earlier stages of
growth, when it is provided with a stem, at once shows its relation to
the old stemmed Crinoids, the earliest representatives of the class of
Echinoderms.
These coincidences are still more striking among living animals, where
they can be more readily and fully traced, and often give us a key to
their relative standing, which our knowledge of their anatomical
structure fails to furnish. This is perhaps nowhere more distinctly
seen than in the type of Radiates, where the Acalephs in their first
stages of growth, that is, in their Hydroid condition, remind us of
the adult forms among Polyps, showing the structural rank of the
Acalephs to be the highest, since they pass beyond a stage which is
permanent with the Polyps; while the adult forms of the Acalephs have
in their turn a certain resemblance to the embryonic phases of the
class next above them, the Echinoderms. Within the limits of the
classes, the same correspondence exists as between the different
orders; the embryonic forms of the higher Polyps recall the adult
forms of the lower ones, and the same is true of the Acalephs as far
as these phenomena have been followed and compared among them. In the
class of Echinoderms the comparison has been carried out to a
considerable extent, their classification has hitherto been based
chiefly upon the ambulacral system, so characteristic of the class,
but so unequally developed in the different orders. This places the
Holothurians, in which the ambulacral system has its greatest
development, at the head of the class; next to them come the
Sea-urchins or Echinoids; then the Star-fishes; then the Ophiurans and
Crinoids, in which the ambulacral system is reduced to a minimum.
Another basis for classification in this type, which gives the same
result, is the indication of a bilateral symmetry in some of the
orders. In the Holothurians, for instance, there is a decided tendency
toward the establishment of a posterior and anterior extremity, of a
right and left, an upper and lower side of the body. In the
Sea-urchins, in many of which the mouth is out of centre, placed
nearer one side than the other, this tendency is still apparent, while
in the three lower groups, the Star-fishes, Ophiurans, and Crinoids,
it is almost entirely lost, in the equal division of identical parts
radiating from a common centre. A comparison of the embryonic and
adult forms in these orders, confirms entirely this classification
based upon structural features. The Star-fishes, in their earlier
stages, resemble the mature Ophiurans, while the Crinoids, the lowest
group of all, retain throughout their whole existence many features
characteristic of the embryonic conditions of the higher Echinoderms.
In this principle of classification, already so fertile in results, we
may hope to find, in some instances, the solution of many perplexing
points respecting the structural rank of animals, the confirmation of
classifications already established; in others, an insight into the
true relations of groups which have hitherto been divided upon purely
arbitrary grounds.
* * * * *
DISTRIBUTION OF LIFE IN THE OCEAN.
We have seen that while our bay is rich in certain species, it is
wholly deficient or but scantily supplied with others, and that the
character of the animals inhabiting its waters is more or less
directly connected with general physical conditions. Such an area,
limited though it be, gives us some insight into the laws which, in
their wider application, control the distribution of marine life along
the shores of the most extensive continents. The coast of
Massachusetts, taken as a whole, is like that of New England
generally, a rocky coast; yet it has its sandy and muddy beaches, and
though it lies for a great part open to the sea, it has nevertheless
its sheltered harbors, its quiet bays and snug recesses.
A comparison of these limited localities with far more extensive
reaches of shore, where similar physical conditions prevail, shows
that they reproduce, in fainter and less various characters of course,
in proportion to their narrower boundaries, but still with a certain
fidelity, the same combinations of animal and vegetable life. In other
words, a sandy beach, however small, gives us some idea of the nature
of the animals we may look for on any sandy coast, as, for instance,
clams of various kinds, razor-shells, quahogs, snails, &c., creatures
who can penetrate the sand, drag themselves through it or over it,
leaving their winding trails as they go, and to whom the conditions
prevailing in such spots are genial. So the narrowest mud flat on the
sea-shore or muddy beach will give us the same dead and inanimate
aspect which characterizes a more extensive coast of like character,
where the gases always generated in mud are deadly to many kinds of
animals. The beings who find a home in such localities are of closely
allied species, chiefly a variety of worms, who burrow their way into
the mud, and seem to court the miasma so fatal to other creatures. The
same is true of any stony beach or rocky shore not more than a quarter
of a mile in length; it gives us an idea of the animal population on
any similar coast of greater extent.
These correspondences are of course modified by differences in
climatic conditions. The animals on a sandy beach or a rocky shore, on
the coast of Great Britain, for instance, are not absolutely identical
with those of a sandy beach or a rocky shore on the coast of New
England, but they are more or less nearly related to them. Naturalists
refer to this reiteration, all the world over, of like organic
combinations under similar circumstances, when they speak of
"representative species." The aggregate result is the same, though the
individual forms are slightly modified. And here lies one secret of
the infinite variety in nature, by which the old seems ever new, and
the same thought has an eternal freshness and originality, endlessly
repeated, yet never hackneyed.
In this sense our bay presents, on a miniature scale, a variety of
physical and organic combinations, which may be compared to those more
extensive divisions in the geographical distribution of animals and
plants, called by naturalists zoölogical or botanical provinces or
districts, the animal and vegetable populations of which are
technically designated as their faunæ and floræ. Such organic realms,
as we may call them, have long been recognized on land, and the most
extensive among them are easily distinguished. No one will fail to
recognize the tropical zone, with its royal dynasty of palms and all
the accompanying glories of a tropical vegetation, its birds of
brilliant plumage, its large Mammalia, lions, tigers, panthers,
elephants, and its great rivers haunted by gigantic reptiles. Nor is
the representation of vegetable and animal life less characteristic in
the temperate zone, where the oak is monarch of the woods, with all
his attendant court of elms, walnuts, beeches, birches, maples, and
the like, where birds of more sober hues, but sweeter voices, take the
place of the brilliant parrots and many-tinted humming-birds of the
tropical forest; while buffaloes, bears, wolves, foxes, and deer
represent the larger Mammalia. In the arctic zone, though marked by
peculiar and distinctive features, vegetation has dwindled to a
minimum; the birds are chiefly gulls and ducks, which go there for the
breeding season in the summer, and the reindeer and polar bears are
almost sole possessors of the snow and ice-fields; but this meagreness
in the representation of the larger land Mammalia is amply compensated
in the numbers of heavy aquatic Mammalia, the whales, walruses, seals,
and porpoises of the Arctic seas.
During the last half-century, since the geographical distribution of
animals and plants has become a subject of more careful investigation
among naturalists, these broad zones of the earth's surface, with
their characteristic populations and vegetation, have been subdivided,
according to more limited and special combinations of organic forms,
into narrower zoölogical and botanical areas. The application of these
results to marine life is however of much more recent date, and indeed
it would seem at first sight, as if the water, from its own nature,
could hardly impose a barrier so impassable as the land. The
localization of the marine faunæ and floræ is nevertheless as distinct
as that of terrestrial animals and plants, and late investigations
have done much to explain the connection of this distribution with
physical conditions.
A glance at the coast of our own continent, starting from the high
north and making the circuit of its shores, from Baffin's Bay to
Behring's Straits, will show us to what a variety of physical
influences the animals who live along its shores are subjected. On the
shores of Baffin's Bay, especially on the inner coast of Greenland,
where the glaciers push their way down to the very brink of the water,
and annually launch their southward-bound icebergs, we shall hardly
expect to find a very abundant littoral fauna. On its western shore,
where the ice does not advance so far, and a greater surface of rock
is exposed, the circumstances are more favorable to the development of
animal life. Here abound the winged Mollusks (Pteropods), often swept
down to the coast of Nova Scotia by the cold current from Baffin's
Bay; the "whale feed," as the fishermen call them, because the whales
devour them voraciously. Here occur also many compound Mollusks,
especially a variety of Ascidians, and the highly colored stocks of
Bryozoa. With them is found the Comatula of the northern waters, one
of the few modern Crinoids, and beside these a number of Star-fishes,
Sea-urchins, and Holothurians, not differing so essentially from those
already described as to require special mention.
Along the shore of Labrador and Newfoundland, the coast is wholly
rocky, and especially about Newfoundland it is deeply indented with
bays. Here there is ample opportunity for the growth of certain kinds
of animals in sheltered nooks. The number of species is, however, much
greater along the shores of Maine, Nova Scotia, and New Brunswick than
in Labrador, owing no doubt to the milder climate. The beautiful shore
of Maine, with its countless islands, and broken, picturesque outline,
is very rich in species. Parts of this coast are remarkable for a
variety of naked Mollusks, as well as for the great numbers of
bright-colored Actiniæ, and also for the more brilliant kinds of
Holothurians, the Cuvieria, and the like. The latter are especially
abundant in the Bay of Fundy, and here also occurs the only Northern
representative on our coast of the Sea-fans or Gorgoniæ, so common on
the shores of Florida.
Farther south, from Cape Cod to Cape Hatteras, the character of the
coast changes; it becomes more sandy, and though here and there the
aspect is varied by a rocky promontory or a stony beach, yet the
general character is flat and sandy. With this new character of the
shore, the fauna is also greatly modified, and it is worthy of remark,
that while thus far the representative species have reflected the
character of animals to the north of them, they now begin to represent
rather those of the Carolina shores. South of Cape Cod come in a kind
of Scallop and Periwinkle, very different from the larger Scallops
found on the coast of Maine and the British Provinces; our Sea-urchin
is replaced by the Echinocidaris, with its few long spines, and an
entirely new set of Crustacea and Worms make their appearance on this
more sandy bottom. And here we must not forget that not only is the
aspect of the animal life changed, as we pass from a rock-bound to a
sandy coast, but that of the vegetation also. The various many-tinted
sea-weeds of the rocky shore disappear almost entirely, and their
place is but poorly supplied by the long eel-grass, which is almost
the only marine plant to be found in such a locality. Beside its more
sandy character, the coast from Cape Cod to Cape Hatteras is affected
by the large amount of fresh water poured into the sea along its whole
line, greatly modifying the character of the shore animals. The
Hudson, the Delaware, the Susquehanna, the Potomac, the James, the
Roanoke, and the large estuaries connected with some of these rivers,
give a very peculiar character to the shore, and bring down, not only
a vast supply of fresh water, but also a large quantity of detritus of
all sorts from the land. Under these circumstances life would be
impossible for many of the animals which live farther north. The only
locality on the North Atlantic shore, where the conditions are
somewhat similar, is at the mouth of the St. Lawrence, that great
drainage-bed through which the Canadian lakes empty their superfluous
waters into the Gulf of St. Lawrence.
The whole coast of the Carolinas, from Cape Hatteras to Florida, is a
sandy beach; but though in this respect it resembles that immediately
to the north of it, it differs greatly in other features.
Comparatively little fresh water is poured into the ocean along this
shore, and its more southerly range, instead of being protected by
sand-spits like Pamlico and Albemarle Sounds, or broken by estuaries
and inlets like the coast of Virginia, lies broadly open to the sea.
On its extensive beaches we have the large Pholas, burrowing deep
below the surface, and the Cerianthus, those long, cylindrical
Actiniæ, enclosed in sheaths, with their bright crowns of
gayly-colored tentacles; the free colonies of Halcyonoids abound also
on this coast, and a new set of Sea-urchins (Spatangoids and
Clypeastroids) make their appearance.
Farther south, along the Florida coast, a new element comes in, that
of the coral reefs, enclosing shallow channels near the shore, and
thus providing sheltered harbors on their leeward side, while on their
seaward side they slope steeply to the ocean. Beside this, the reef
itself affords a home for a great variety of creatures, who bore their
way into it and live in its recesses, as some insects live in the bark
of trees. Perhaps a more favorable combination of circumstances for
the development of marine life does not exist anywhere than about the
coral reefs of Florida, and certainly nowhere is there a more rich and
varied littoral fauna, especially on their western shore within the
Gulf of Mexico. Here swims the Portuguese Man-of-War, borne gayly
along on the surface of the water by its brilliant float, here the
blue Velella sets its oblique sail to the wind, and hosts of the
lighter and more brightly tinted corals fringe the shore with a
many-colored shrubbery. In these waters are also found the blue and
yellow Angel-fish, the Parrot-fish (Scarus), and the strange
Porcupine-fish (Diodon). Vegetable life is comparatively scanty in
these tropical waters, where there are scarcely any sea-weeds, except
the corallines or limestone Algæ of the reefs. The shore of the Gulf
of Mexico, as a whole, has much the same character as that of the
Carolinas, until we reach the point where the mountains and plateau of
Mexico come down to the coast. From this point to the Isthmus of
Panama the coast is again rocky.
Crossing the Isthmus and following the Pacific shore of the continent
northward, we find a sandy open shore alternating with rocky beaches
as far north as Acapulco. Along this coast there is to be found a
great variety of corals, especially Sea-fans, growing on the rocks,
but no reef. The Pocillopora, an Acalephian coral, the Pacific
representative of the Millepore of Florida, is especially abundant. On
the peninsula of Lower California we come again upon a rocky coast,
with steep bluffs, extending into the sea. Within the Gulf of
California are found, on its sandy coast, peculiar kinds of
Sea-urchins, Spatangoids, and Clypeastroids, which occur nowhere else
on this coast. From Cape St. Lucas up to the Straits of Fuca, with the
exception of the large estuary forming the Bay of San Francisco, there
are scarcely a couple of harbors of any consequence. The whole shore
is most inhospitable, and the violent northwest winds in summer, and
the southeast winds in winter, render it still more bleak and
difficult of approach. In consequence of these conditions, the fauna
is scanty along a great part of the shore; the best spots for
collecting are the beaches, near the head of the peninsula, opposite
the islands of Santa Barbara and San Diego, and that within the harbor
of San Francisco. On the former, large Craw-fishes abound (Palinurus),
akin to those of Florida, though specifically different from them. In
the latter, the great amount of fresh water prevents the fauna from
being exclusively marine; this harbor is, nevertheless, the great
centre of the viviparous fishes, and contains also a large variety of
peculiarly shaped Sculpins.
Farther north, between the Straits of Fuca and the island of Sitka,
the shore resembles that of Maine, with its many islands, bays, and
inlets; a succession of long, narrow islands forms a barrier along the
coast, enclosing the shore waters, so as almost to make them into an
inland sea. But little fresh water empties upon this part of the
coast, and here, where the salt water is little modified by any
deposit from the land, but where the violence of the ocean is broken
by this barrier of islands, there is a full development of marine
life. The shores of the Gulf of Georgia, and those of Vancouver's
Island, seem to be especially the home of the Star-fishes. The fauna
of this locality has been but little investigated, and yet the number
of species of Star-fishes known from there is greater than from any
other region; many of them are of colossal size, measuring some four
feet in diameter. This coast seems also very favorable for the
development of Hydroids, in consequence of which its waters swarm with
a variety of Jelly-fishes. The Pennatula, that pretty compound
Halcyonoid, with its feather-like sprays, is another characteristic
type of this fauna. Beyond this, from Sitka to Behring's Straits, the
same rocky coast prevails as in Labrador and Greenland. In Behring's
Straits we return again to the forests of beautiful compound Mollusks,
or rather to a variety of "representative species," resembling the
Bryozoa and Ascidians so abundant in Baffin's Bay. The depth of the
water, however, is much less here than on the corresponding Atlantic
coast, where, south of Greenland, along the shore of Labrador, the
water is very deep, while in Behring's Straits the depth is not
greater than from one hundred to one hundred and twenty fathoms. The
respective faunæ of these two shores are also affected by the
difference of temperature, the cold current from Baffin's Bay sweeping
down upon the coast of Labrador, while, through Behring's Straits, the
warm current from the Pacific pours into the Arctic Ocean.
Thus the whole coast of our continent is peopled more or less thickly
with animals. But now arises a new set of inquiries; how far into the
sea do these animals extend? how wide is their domain? Do they wander
at will in the ocean, or are they bound by any law to keep within a
certain distance of the shore? These questions would seem to be easily
answered, for wherever we go on the surface of the sea, and as far as
the eye can penetrate into its depths, we find it full of life; and
yet a closer examination shows that all these beings have their
appointed boundaries. Along the shores, animal and vegetable life
seems to be distributed in certain definite combinations. Those who
are familiar with rocky beaches readily recognize the different bands
of color produced by the various kinds of sea-weed growing at given
distances between high and low-water-mark. First comes the olive green
rockweed (the Fucus), and with it are found barnacles and small
Crustacea, myriads of which are to be seen hopping about in this
rockweed when the tide is out. Below these are the brown crispy
Rhodersperms and Melanosperms, and associated with them are
Star-fishes, Crabs, and Cockles. Next in order is the Laminarian zone.
Here we have the broad fronds of the Laminaria, the "devil's aprons,"
as the fishermen call them; in this zone is the home of the
Sea-urchin, and here will be found also a few small fishes. Lastly we
have the Coralline zone, so called on account of the lime deposit in
the sea-weeds, giving them the rigidity of corals; among these the
Lobsters make their appearance, and here are to be found also numerous
clusters of Hydroids, the nurses of the Jelly-fishes.
This distribution is not casual; these belts of animal and vegetable
life are sharply defined and so constantly associated, that they must
be controlled by the same physical laws. The first important
investigations on this subject were made by Örsted, the distinguished
Danish naturalist. He undertook a complete topographical survey of the
coast near which he lived, carrying his soundings to a depth of some
twelve fathoms, and found that both the fauna and flora of the shore
were divided, according to the depth of the water, into bands of
vegetable and animal life, corresponding very nearly with those given
above. His observations were, however, limited, not extending beyond
the neighborhood of his home. It is to Edward Forbes, the great
English naturalist, whose short life was so rich in results for
science, that we owe a more complete and extensive investigation of
the whole subject.
[Illustration: Diagram of a rocky beach.]
Aided by a friend, Captain McAndrew, who placed his yacht at his
disposal, he made a series of observations on the British,
Scandinavian, and Danish coasts, and explored also with the same
object the shores of the Mediterranean. Not content with sounding the
present ocean, he sunk his daring plummet in the seas of past
geological ages, and by comparing the nature and position of their
fossil remains with those of living marine faunæ, he measured the
depths of the water along their shores. He collected a vast amount of
material, and the results of his labors have formed the basis of all
subsequent generalizations upon this subject. Nevertheless he arrived
at some erroneous conclusions, which, had he lived, he would no doubt
have been the first to correct. Dredging from low-water-mark outward,
he found that, from the Laminarian and Coralline zone, the animals
began gradually to decrease in number, and that, at a depth of two or
three hundred fathoms, the dredge always came up nearly empty. He
inferred that at a certain depth the weight of water became too great
to be endured by animals, and that the ocean beyond this line, like
the land beyond the line of perpetual snow, was barren of life. This
result seemed the more probable on account of the immense pressure to
which animals are subjected, even at a comparatively moderate depth. A
column of water thirty-two feet high is equal to one atmosphere in
weight; this pressure being increased to the same amount for every
thirty-two feet of depth, it follows that a fish one hundred and
twenty-eight feet, or some twenty fathoms below the surface, is under
the pressure of almost four atmospheres plus that of the air outside.
Wherever tides run high, as in the Bay of Fundy, for instance, where
an animal is under the pressure of one atmosphere at low tide, and of
three atmospheres at high tide, we see that marine animals are
uninjured by great changes of pressure. Yet it seems natural to
suppose that there is a limit to this power of resistance; and that
there must exist barren areas at the bottom of the ocean, as destitute
of life as the regions on the earth which are above the line of
perpetual snow. No doubt pressure does influence the distribution of
life in the ocean; but it would seem, from subsequent observations,
that the boundaries assigned by Forbes were far too narrow, and that
the structure of many marine animals enables them to live under a
weight, the one hundredth part of which would be fatal to any
terrestrial animal.
For some years Forbes's theory was very generally accepted, and the
results of Darwin's and Dana's investigations, showing that corals
could not live beyond a depth of fifteen fathoms, seemed to confirm
it. But, quite recently, facts derived from new and unlooked-for
sources of information have given a check to this theory. Commerce has
come to the aid of science (rewarding her for the gift first received
at her hands), and the telegraph cables, alive with the secrets of sea
and land, have brought us tidings from the deep. Dr. Wallich, the
naturalist who in 1860 accompanied the expedition to explore the bed
of the Atlantic, previous to laying the telegraphic cable, first
called attention again to this subject. He brought up various animals,
highly organized, from a depth of about nineteen hundred fathoms.
Yet, in spite of this positive evidence added to the former
observations of Ehrenberg, and to those of Sir James Ross, who, in the
Antarctic Sea, brought up an Euryale on a sounding-line from a depth
of eight hundred to a thousand fathoms, naturalists were slow to
believe that the distribution of animal life in the ocean was not
limited to the shallow depths assigned by Edward Forbes. In the
Mediterranean and in the Red Sea, from depths of eighteen hundred to
two thousand fathoms, living animals have been brought up on the
telegraph wires, not of doubtful infusorial character, hovering on the
border-land between animal and vegetable life, but of considerable
size, as, for instance, one or two kinds of Crustacea, Cockles, stocks
of Bryozoa and tubes of Annelids. When the cable between France and
Algiers was taken up from a depth of eighteen hundred fathoms, there
came with it an Oyster, Cockle-shells, Annelid tubes, Bryozoa and
Sea-fans. As these animals were growing upon it, there could be no
doubt that they had their normal life and development at this depth,
and since they are carnivorous, they tell also of the existence of
other animals with them on which they feed.
The dredge, which thus far has played an important part in zoölogical
researches, is destined to revolutionize many of our accepted
theories, if we can judge of its future by the brilliant results of
the last few years.
From 1861 to the present time the Swedish government has sent several
expeditions to Spitzbergen and Greenland. They carried on dredging
operations most successfully to a depth of twenty-six hundred fathoms.
For some years past Lovén, Koren, and Danielssen, the elder and
younger Sars, and other Scandinavian naturalists, have made systematic
dredgings along the coast of Norway, which, though not extending below
four hundred fathoms or thereabouts, have yet furnished most
astounding results.
The United States Coast Survey has, in connection with an exploration
of the Gulf Stream, been the first to establish a systematic series of
dredgings at great depths, continued during several years. The results
have proved conclusively that there exists everywhere, in the deep
sea, modified, of course, according to the nature of the bottom and
the temperature, a most varied fauna, totally distinct from that
characteristic of the shores and of shallower waters. Since 1867 Count
Pourtales has had charge of these investigations, first established
many years ago by Professor Bache and continued by his successor
Professor Peirce. He has dredged across the Gulf Stream between
Florida and Cuba to a depth of about seven hundred fathoms, collecting
an immense number of marine animals entirely unknown before, and
characteristic of the different belts of depth, having a most
extraordinary geographical distribution, many of the species being
found in Florida, the Azores, the Faroe Islands, and the west coast of
Norway.
The English Admiralty has for two summers detailed a vessel admirably
fitted for such purposes, intrusting the scientific direction of the
expedition to Dr. Carpenter, Professor Thomson, and Mr. Jeffreys.
Their dredgings, carried on to the enormous depth of two thousand four
hundred and thirty-five fathoms, have in every respect corroborated
the conclusions drawn from the collections made by Count Pourtales and
the Scandinavian naturalists, who, not content with so thoroughly
exploring their own coast, have even sent a ship of war to dredge
across the whole Atlantic.
These discoveries only show how much yet remains to be done before we
shall fully understand the laws of marine life. But we already have
ample evidence that the same beneficent order controls the
distribution of animals in the ocean as on land, appointing to all its
inhabitants their fitting home in the dim waste of waters.
* * * * *
SYSTEMATIC TABLE OF THE ANIMALS DESCRIBED IN THIS VOLUME
RADIATA CUV.
CLASS I.--POLYPI LAM.
ORDER I.--ACTINARIA EDW.
_Metridium marginatum_ EDW. _Rhodactinia Davisii_ AG. _Bicidium
parasiticum_ AG. _Arachnactis brachiolata_ A. AG. _Halcampa albida_ AG.
ORDER II.--MADREPORIA AG.
_Astrangia Danæ_ AG.
ORDER III.--HALCYONARIA EDW.
_Halcyonium carneum_ AG.
CLASS II.--ACALEPHÆ CUV.
ORDER I.--HYDROIDEA JOHNST.
_Velella mutica_ BOSC. _Physalia Arethusa_ TIL. _Nanomia cara_ A. AG.
_Millepora alcicornis_ LIN. _Hydractinia polyclina_ AG. _Tubularia
Couthouyi_ AG. _Hybocodon prolifer_ AG. _Coryne mirabilis_ AG. _Turris
vesicaria_ A. AG. _Bougainvillia superciliaris_ AG. _Dysmorphosa
fulgurans_ A. AG. _Dynamena pumila_ LAMX. _Dyphasia rosacea_ AG. _Lafoea
cornuta_ LAMX. _Melicertum campanula_ PÉR. et LES. _Ptychogena lactea_
A. AG. _Laomedea amphora_ AG. _Zygodactyla groenlandica_ AG. _Tima
formosa_ AG. _Eucope diaphana_ AG. _Clytia bicophora_ AG. _Oceania
languida_ A. AG.
ORDER II.--DISCOPHORÆ ESCH.
_Haliclystus auricula_ CLARK. _Trachynema digitale_ A. AG. _Campanella
pachyderma_ A. AG. _Cyanea arctica_ PÉR. et LES. _Aurelia flavidula_
PÉR. et LES.
ORDER III.--CTENOPHORÆ ESCH.
_Idyia roseola_ AG. _Pleurobrachia rhododactyla_ AG. _Bolina alata_ AG.
CLASS III.--ECHINODERMATA KLEIN.
ORDER I.--CRINOIDEA MILL.
_Pentacrinus,_ _Alecto Eschrichtii_ M. & T. _Alecto meridionalis_ AG.
ORDER II.--OPHIURIDEA FORBES.
_Amphiura squamata_ SARS. _Ophiopholis bellis_ LYM. _Astrophyton
Agassizii_ STIMP.
ORDER III.--ASTERIDEA FORBES.
_Ctenodiscus crispatus_ D. & K. _Hippasteria phrygiana_ AG. _Cribrella
oculata_ FORBES. _Solaster endeca_ FORBES. _Crossaster papposa_ M. & T.
_Astracanthion pallidus_ AG. _Astracanthion berylinus_ AG.
ORDER IV.--ECHINIDEA LAM.
_Toxopneustes drobachiensis_ AG. _Echinarachnius parma_ GRAY.
ORDER V.--HOLOTHURIDEA BRANDT.
_Caudina arenata_ STIMP. _Synapta tenuis_ AYRES. _Cuvieria squamata_ D.
& K. _Pentacta frondosa_ JÄG.
* * * * *
INDEX.
PAGE
Abbreviations of authors' names, xii
Acalephs, 21
Actinia, 7
Actinoids, 7
Alecto Eschrichtii, 121
Alecto meridionalis, 121
Amphiura squamata, 115
Arachnactis brachiolata, 14
Astracanthion berylinus, 108
Astracanthion pallidus, 112
Astrangia Danæ, 16
Astrophyton Agassizii, 117
Aurelia flavidula, 42
Bicidium parasiticum, 15
Bolina alata, 31
Bougainvillia superciliaris, 69
Campanella pachyderma, 44
Campanularians, 49
Caudina arenata, 97
Circe, 45
Clytia bicophora, 56
Comatula, 121
Coryne mirabilis, 68
Cribrella oculata, 112
Crinoids, 120
Crossaster papposa, 114
Ctenodiscus crispatus, 113
Ctenophoræ, 26
Cuvieria squamata, 98
Cyanea aretica, 38
Development of Melicertum, 64
'' '' Tima, 64
Discophoræ, 37
Distribution of Life in the Ocean, 141
Dynamena pumila, 66
Dyphasia rosacea, 67
Dysmorphosa fulgurans, 75
Echinarachnius parma, 106
Echinoderms, 91
Echinoids, 101
Embryology of Astracanthion, 124
'' '' Ctenophoræ, 34
'' '' Echinoderms, 123
'' '' Ophiurans, 135
'' '' Sea-urchins, 130
'' '' Star-fishes, 124
Eucope diaphana, 50
Halcampa albida, 16
Halcyonium carneum, 19
Halcyonoids, 19
Haliclystus auricula, 46
Hippasteria phrygiana, 113
Holothurians, 95
Hybocodon prolifer, 74
Hydractinia polyclina, 73
Hydroids, 49
Idyia roseola, 32
Lafoea cornuta, 67
Laomedea amphora, 65
Lucernaria, 46
Madreporians, 16
Melicertum campanula, 63
Metridium marginatum, 7
Millepora alcicornis, 22
Mode of catching Jelly-fishes, 85
Nanomia cara, 76
Oceania languida, 53
Ophiurans, 115
Ophiopholis bellis, 115
Pentacrinus, 121
Pentacta frondosa, 99
Pleurobrachia rhododactyla, 27
Polyps, 5
Ptychogena lactea, 86
Physalia Arethusa, 83
Radiates, 1
Rhodactinia Davisii, 13
Sarsia, 68
Sea-urchin, 101
Sertularians, 66
Solaster endeca, 114
Star-fishes, 108
Synapta tenuis, 95
Systematic Table, 152
Tima formosa, 60
Toxopneustes drobachiensis, 101
Trachynema digitale, 45
Tubularia Couthouyi, 72
Tubularians, 67
Turris vesicaria, 69
Velella mutica, 84
Zygodactyla groenlandica, 57
THE END.
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Cambridge: Stereotyped and Printed by Welch, Bigelow, & Co.
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Transcriber's Notes
With the exception of the corrections detailed below and some
minor corrections (missing periods, commas, etc.) which were made
but are not listed, the text presented is that in the original
printed version. The illustration labels were originally presented
at the bottom of the containing page; but have been moved beneath
the illustrations.
Typographical Corrections
Page xi: Ophiopolis => Ophiopholis
Page 4: diferent => different
Page 21, 101, 127, 131, 148: so called => so-called
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