Fig. 151. Astrophyton, Basket-fish; in a natural attitude.Fig. 151. Astrophyton, Basket-fish; in a natural attitude.Click on image to view larger size.
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 froma 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.
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 aMackerel. 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 fitterEnglish/name be found for it, why may it not be called (in regard of what hath been before mentioned of it) aBasket-Fish, or aNet-Fish, or aPurs-net-Fish?" And so it remains to this day as the Governor of Connecticut first christened it, the Basket-fish.
CRINOIDS.
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.
[fig 152]
Fig. 152. Fossil Pentacrinus.Fig. 152. Fossil Pentacrinus.
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 morerecent periods that we find the free Crinoids, corresponding to the Comatula.
Comatula. (Alecto meridionalisAg.)
One large species of Comatula (Alecto EschrichtiiM. & 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 fromFig. 153) represents a group of the young of the Charleston Comatula, still attached to the parent body by their stems, and in various stages ofdevelopment. 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.
[fig 153]
[fig 154]
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. InFig. 153, a faint star-like dot (y) may be seen attached to the side of the disk by a slight line. InFig. 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 internalstructure 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.)
[fig 155]
[fig 156]
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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 (ma,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 isan 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 (ww',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 weshall 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.
[fig 158]
[fig 159]
[fig 160]
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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 inFig. 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.)
[fig 165]
Fig. 165. Larva in which arms are developing, lettering as before; e' e'' e''' e^4 e^5 e^6 arms, o oesophagus.Fig. 165. Larva in which arms are developing, lettering as before;e' e'' e'''e4e5e6arms,ooesophagus.
On the surface where the mouth is formed, and very near it on either side, two small arcs arise, asvinFig. 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.
[fig 166]
[fig 167]
fig. 166Fig. 166. Adult larva,so-calledBrachiolaria,lettering as before;rback of young Star-fish,ttentacles of young Star-fish,ff'brachiolar appendages.Click on image to view larger size.fig. 167Fig. 167. Fig. 166 seen in profile, lettering as before.
Fig. 166. Adult larva,so-calledBrachiolaria,lettering as before;rback of young Star-fish,ttentacles of young Star-fish,ff'brachiolar appendages.
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Fig. 167. Fig. 166 seen in profile, lettering as before.
[fig 168]
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Fig. 168. Star-fish which has just resorbed the larvaFig. 168. Star-fish which has just resorbed the larva, seen from the back;bmadreporic opening.Fig. 170. Young Star-fish which has become symmetrical, seen from the backFig. 170. Young Star-fish which has become symmetrical, seen from the back;t'odd tentacle.Fig. 169. Fig. 168, seen from the mouth side; m mouth, t tentacles.Fig. 169. Fig. 168, seen from the mouth side;mmouth,ttentacles.
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 thebeginning 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 (ff',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. 168represents 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. 169represents the lower surface at the same moment, with the open mouth (m), around which the tentacles (t) are just beginning to appear; whileFig. 170shows 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. 172represents 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.
[fig 171]
[fig 172]
[fig 173]
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Sea-urchins.
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 ofgrowth. In these early stages the young, or theso-calledlarvæ 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 inFig. 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 withFigs. 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. InFig. 176the 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. 177shows 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. InFig. 178the Plutens is already in process of absorption; inFig. 179it has wholly disappeared; inFigs. 180and 181we 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.
[fig 176]
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Ophiurans.
Fig. 183represents 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. 182represents 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.
[fig 182]
[fig 183]
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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 ofFig. 168of 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. 185represents 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 animalkingdom, 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 equaldivision 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.