Seaside Studies in Natural History Part 10

_Star-fish_. (_Astracanthion_.)

[Ill.u.s.tration: Fig. 155. Egg of Star-fish.]

[Ill.u.s.tration: Fig. 156. Egg of Star-fish in which the yolk has been divided into two segments.]

[Ill.u.s.tration: 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 s.p.a.ce within, and by the time the segmentation is completed, they form a continuous spherical sh.e.l.l. 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 a.n.u.s 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 a.n.u.s. 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.

[Ill.u.s.tration: Fig. 158. Larva just hatched from egg; a thickened pole.]

[Ill.u.s.tration: Fig. 159. Larva somewhat older than Fig. 158; _m_ _a_ depression at thickened pole.]

[Ill.u.s.tration: Fig. 160. Larva where the depression has become a digestive cavity _d_, opening at _a_.]

[Ill.u.s.tration: Fig. 161. Earlets, _w_ _w'_ (water-tubes), developed at the extremity of the digestive cavity _d_; _m_ mouth.]

[Ill.u.s.tration: Fig. 162. More advanced larva; _a_ _d_ _c_ digestive system, _v_ vibratile chord, _m_ mouth.]

[Ill.u.s.tration: Fig. 163. Profile view of larva; _b_ madreporic opening, _w'_ earlet, _a_ _d_ digestive system, _m_ mouth, _v_ _v'_ vibratile chord.]

[Ill.u.s.tration: 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 ca.n.a.l. 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.)

[Ill.u.s.tration: 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 s.h.i.+elds or plastrons. (Figs. 165, 166.) The corners of these s.h.i.+elds 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.

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: Fig. 167. Fig. 166 seen in profile, lettering as before.]

[Ill.u.s.tration: Fig. 168. Star-fish which has just resorbed the larva, seen from the back; _b_ madreporic opening.]

[Ill.u.s.tration: Fig. 169. Fig. 168, seen from the mouth side; _m_ mouth, _t_ tentacles.]

[Ill.u.s.tration: 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 a.s.sumed 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 circ.u.mference, 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 pedicellariae 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 a.s.sumed its ultimate outline, it lacks some features of the adult, having only two rows of tentacles, whereas the full-grown Star-fish has four.

[Ill.u.s.tration: Fig. 171. Lower side of ray of young Star-fish; _m_ mouth, _b_ madreporic body, _e_ eye-speck.]

[Ill.u.s.tration: Fig. 172. Young Star-fish seen in profile; _t'_ odd tentacle at extremity of arm.]

_Sea-urchins_.

[Ill.u.s.tration: Fig. 173, 174, 175. Young larvae 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 a.n.a.lyzed 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 larvae 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 att.i.tude; 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.

[Ill.u.s.tration: Fig. 176. Adult larva of Toxopneustes, _f_ brachiolar appendages.]

[Ill.u.s.tration: Fig. 177. Fig. 176 seen endways.]

[Ill.u.s.tration: Fig. 178. The Sea-urchin resorbing the arms of the larva.]

[Ill.u.s.tration: Fig. 179. Half a young Sea-urchin immediately after resorption of the larva; _s"_ _s"_ spines, _t'_ _t'_ ambulacral tentacles.]

[Ill.u.s.tration: Fig. 180. Young Sea-urchin older than Fig. 179; _t_ _t'_ tentacles, _s"_ _s"'_ spines.]

[Ill.u.s.tration: Fig. 181. Still older Sea-urchin; _t_ _t_ tentacles, _a_ a.n.u.s, _p_ pedicellariae; sh.e.l.l 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.

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: Fig. 184 Young Ophiuran which has resorbed the whole larva; _r_ middle plate of back.]

[Ill.u.s.tration: Fig. 185 Cl.u.s.ter 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, pa.s.sing 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 cl.u.s.ter 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 circ.u.mstances 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 s.p.a.ce in which they have to move; and the water-tubes on which the upper and lower surfaces of the body arise, the s.h.i.+elds, 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 pa.s.sage 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 b.u.t.terfly, is also distinct and definitely marked; the moment when the animal pa.s.ses 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 pa.s.s 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 cla.s.sification 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 cla.s.s 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 pa.s.s 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 cla.s.s next above them, the Echinoderms. Within the limits of the cla.s.ses, 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 cla.s.s of Echinoderms the comparison has been carried out to a considerable extent, their cla.s.sification has. .h.i.therto been based chiefly upon the ambulacral system, so characteristic of the cla.s.s, 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 cla.s.s; 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 cla.s.sification in this type, which gives the same result, is the indication of a bilateral symmetry in some of the orders. In the Holothurians, for instance, there is a decided tendency toward the establishment of a posterior and anterior extremity, of a right and left, an upper and lower side of the body. In the Sea-urchins, in many of which the mouth is out of centre, placed nearer one side than the other, this tendency is still apparent, while in the three lower groups, the Star-fishes, Ophiurans, and Crinoids, it is almost entirely lost, in the equal division of identical parts radiating from a common centre. A comparison of the embryonic and adult forms in these orders, confirms entirely this cla.s.sification 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 cla.s.sification, 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 cla.s.sifications already established; in others, an insight into the true relations of groups which have hitherto been divided upon purely arbitrary grounds.

DISTRIBUTION OF LIFE IN THE OCEAN.