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.]