_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 gla.s.s dish filled with Cribrellae might vie with a tulip-bed in gayety and vividness of tints.
[Ill.u.s.tration: 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, ill.u.s.trating 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 pedicellariae on this Star-fish resemble large two-p.r.o.nged clasps, arranged princ.i.p.ally 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 cla.s.s, 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 a.s.sume the deeply indented star-shaped outline, and have suckers at the extremities of the tentacles.
[Ill.u.s.tration: 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 lat.i.tudes, 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.)
[Ill.u.s.tration: Fig. 148. Ophiopholis, from above; natural size.]
There are but two species of the ordinary forms of Ophiurans in Ma.s.sachusetts 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. [Ill.u.s.tration: Fig. 149.
One arm of Fig. 148; from the mouth side.]
[Ill.u.s.tration: 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 s.p.a.ces 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 pa.s.s; 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 s.p.a.ces 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 Aga.s.sizii_ STIMP.)
A singular species of Ophiuran, known among fishermen as the "Basket-fish," (Fig. 151,) is to be found in Ma.s.sachusetts 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, a.s.suming 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 incompa.s.seth 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.
[Ill.u.s.tration: Fig. 151. Astrophyton, Basket-fish; in a natural att.i.tude.]
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.
[Ill.u.s.tration: 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 cla.s.s, and were then so numerous that the cla.s.s of Echinoderms, with only one order, seemed as full and various as it now does with five. The different forms they a.s.sumed 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 sh.o.r.es 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 sh.o.r.es 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 ambulacrae, and on the same side of the disk are situated the mouth and the a.n.u.s; 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.
[Ill.u.s.tration: Fig. 153. Comatula (Living Crinoid) seen from the back; a group of young Comatulae attached to parent.]
[Ill.u.s.tration: Fig. 154. Magnified view of the group of young Comatulae 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 Comatulae, 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 cla.s.s 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 ma.s.s 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 ma.s.s, as in the Acalephs.