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.