Popular Science Monthly/Volume 27/July 1885/Curiosities of Star-Fish Life
|CURIOSITIES OF STAR-FISH LIFE.|
FOR a dozen years past, the eminent English zoölogist, who has become so widely known as an investigator of animal intelligence,
has spent his summers at the sea-side, studying several common forms of marine life. He compares a season's work of this kind Fig. 2.—Pedicellaræ (magnified).
(From Cassell's "Natural History.") to a prolonged picnic, the pleasure of which is accompanied by a sense that no time is being profitlessly spent. Sailing about upon the sunny sea to dip up in muslin nets the creatures at the surface, steaming away far from shore to dredge for other material, and carrying on observations among the tanks and bell-jars of a neat little airy workshop, all have their charms. Even the necessity of devising makeshift apparatus, and of teaching unskilled hands how to help, adds to the enjoyment, as does the overcoming of similar obstacles in a pleasure-excursion. Dr. Romanes has devoted his attention mainly to jelly-fish, star-fish, and sea-urchins, or more particularly to the nervous systems, and the movements controlled by them, in these creatures.
A star-fish, as we all know, consists of a central disk and five radiating arms (Fig. 1). Upon the whole of the upper surface occur numerous calcareous nodules imbedded in the soft flesh, and supporting short spines. One of these nodules, much larger than the others, is always found a little to one side of the center, and is called the madreporic tubercle (Fig. 1, m). With the aid of a lens we may see also on the upper surface a number of small organs, each consisting of a pair of pincers supported on a flexible stalk, scattered about among the calcareous nodules, or attached to the spines; these are known as pedicellaræ (Fig. 2). These organs are provided with muscles' by which the stalk is swayed about, and the pincers are opened and shut. What it is that these curious organs are adapted to seize, and therefore of what use they are in the economy of the animal, has long been a puzzle to naturalists, but Dr. Romanes and his associate, Professor Ewart, have succeeded in throwing some light on this point. In some species of star-fish the size of the central disk is increased so as to fill up the spaces between the rays, the form of the animal thus becoming a pentagon. In other species the reverse process has taken place, the rays having become relatively longer, and, being at the same
time very active, they look like five little snakes joined together by a small circular disk (Fig. 3). Again, in another species the rays branch, these branches again branch, and so on till the animal looks like a mat. Turning now the under surface of our star-fish uppermost, we see that the mouth is in the center of the disk, and that from the mouth radiate five grooves, each extending to the tip of one of the five rays (Fig. 7). On each side of these grooves are many actively moving membranous tubes, which are used for crawling, and are called the pedicels or feet. They are closed at the free end, but communicate by a system of tubes within the body of the animal with the madreporic tubercle. It has been surmised that this tubercle acts as a filter to the sea-water which, with some admixture, forms the liquid circulating in the tubes, and Fig. 4.—The Terminal Portion of a Tube-foot (magnified). Dr. Romanes has proved the surmise to be correct; for colored fluid, injected under pressure into any part of the system of tubes, found its way to the madreporic tubercle, and oozed through its porous substance. The tube-feet are thrust forth or withdrawn by being distended with liquid or emptied. With the exception of a few at the tip of each arm, every tube-foot bears a sucker (Fig. 4); these suckers are pressed closely to a flat surface by filling the tube-feet with liquid; the pressure within the tubes is then lessened, and the greater pressure of the surrounding water holds the suckers fast. They are released by increasing the pressure of the liquid within the tube-feet.
The common star-fish usually crawls in a determinate direction, the feet on the tip of the foremost ray being used as feelers. In a tank, when the star-fish has ascended the side and reaches the surface of the water, it often performs peculiar movements which may be called acrobatic. The animal does not wish to leave its native element—in fact, can not do so, because its sucking feet can act only under water—neither does it wish to descend at once.
It therefore crawls along the side of the tank, now and then throwing back its uppermost ray or rays to feel about for rocks or sea-weed (Fig. 5). If it finds any solid support it will very likely attach its uppermost rays to it, and then, letting go its other attachments, swing from the old support to the new. The activity and co-ordination manifested in these acrobatic movements, says Dr. Romanes, are surprising, and give to the animal an almost intelligent appearance.
The feet of astropecten are partly rudimentary, having lost their terminal suckers, and these star-fish assist themselves in locomotion by the muscular movements of their rays. The brittle-stars are still further removed in the same direction from the common star-fish; their tubular feet are of no use for crawling, while their rays are so long, flexible, and muscular, as to enable them to shuffle quite rapidly over horizontal surfaces. Two opposite arms are used upon the floor with the motion of swimming, the animal leaping forward about two inches at each stroke, and, as these leaps follow one another quickly, the starfish is able to travel at the rate of six feet a minute. A common star-fish can crawl only two inches a minute. Some of the Comatulæ, in which the muscularity of the rays has proceeded still further, are able actually to swim by the co-ordinated movements of their rays.
|Fig. 6.—An Echinus partly denuded of its Spines.(From Cassell's "Natural History.")|
The sea-urchin, or echinus, is a modification of the star-fish structure, having the form of a flattened sphere, and is covered with hard spines (Fig. 6). In the living animal these spines are movable in all directions, each being mounted on a ball-and-socket joint, and provided with muscles at its base. Like the star-fish, the echinus has a madreporic tubercle, pedicellariae, and feet. If we shave off the spines and pedicellariae, we come down to a hard shell, which is hollow and filled with liquid. The liquid resembles sea-water, but is richly corpusculated, and coagulates when exposed to the air. Five double rows of holes extend symmetrically from pole to pole of the shell. It is through these holes that the feet are thrust out, so that in its main features an echinus is merely a star-fish with its five rays curved into the shape of a hollow spheroid, and then converted into a rigid box, with holes left for its feet to come through. The urchin crawls in the same way as the common star-fish, but makes use of its spines also to help push itself along. The suckers, moreover, in being protruded from all sides of a globe instead of from the under side of a flat organism, are of much more use as feelers than they are in the star-fish. If the animal while walking be turned half round, it will continue its movements as before, and hence will proceed in a direction opposite to its former one. When at rest, some of the feet are used as anchors, and others protruded as feelers.
All species of the Echinodermata, when turned upon their backs, are able to right themselves. The brittle-stars can easily perform the
needful manœuvre by wriggling some of their snake-like arms under the inverted disk, and heaving the whole body over by the mere muscularity of these members. The common star-fish, however, experiences more difficulty, and executes the manœuvre mainly by means of its suckers. It twists round the tip of one or more of its rays until the feet there situated are able to get a firm hold of the floor (Fig. 7,
a), then, by successive action of the feet further back in the series, the whole ray is twisted round (b), so that the under surface of the end is applied flat against the floor (c). The semi-turn or spiral then travels on down the ray. Usually two or three adjacent rays perform this manœuvre simultaneously, the spirals of the co-operating rays being invariably turned in the same direction, and, when they have proceeded sufficiently far to drag over the remaining rays, these then abandon their hold on the bottom so as not to offer any resistance to the lifting action of the active rays. The whole movement does not occupy more than half a minute.
But it is in the case of echinus that these righting movements become most interesting, from the fact that they are so much more difficult to accomplish. Two, or perhaps three, adjacent rows of suckers are chosen out of the five to accomplish the task. As many feet in these rows as can reach the floor are thrust downward and fastened firmly to it; by their combined action, as by the pull of liliputian ropes, the globe is tilted slightly in their own direction, the anchoring feet in the opposite rows releasing their hold on the floor to admit of this tilting (Fig. 8). The next feet in the active rows are thus enabled to reach the floor, and, when they have established their hold, they assist in increasing the tilt; then the next feet in the series lay hold, and so on, the globe slowly but steadily rising until it stands upon its equator (Fig. 9). The difficulty of raising such a heavy mass into this
position by means of the slender motive power available is manifest not only from the extreme slowness with which it takes place, but because specimens not perfectly strong may fail completely to reach the position of resting on the equator. Moreover, in some cases when this position has been reached with difficulty, the echinus gives itself a breathing-space, as it were, before beginning its descent. It will be perceived that, as soon as the descent begins, gravity is no longer an obstacle but an aid to the righting movement, and it might be anticipated that the echinus would now simply let go all its attachments and allow itself to roll over into its natural position. But an echinus will never let go its attachments without some urgent reason, seeming to be above all things afraid of being rolled about at the mercy of currents, and therefore it lets itself down almost as slowly as it pulled itself up (Fig. 10).
Single rays separated from a star-fish crawl as fast as the entire animal, and likewise in a determinate direction. They also crawl up
perpendicular surfaces, and when inverted right themselves as quickly as do the unmutilated creatures. A segment of an echinus bearing a single row of ambulacral feet, when propped up on its ab-oral pole, (Fig. 11) will right itself after the manner of entire animals (Fig. 12). It, however, experiences more difficulty in doing so, and very often fails to complete the manœuvre. Such a segment is, of course, analogous to a single detached ray of a star-fish; but on account of the rigid consistence and awkward shape of the segment—standing erect instead of lying flat—it presents a much more curious appearance in locomotion than does the ray of a star-fish.
Dr. Romanes reports observations which show conclusively that the whole external surface, not only of the soft and fleshy star-fish, but even of the hard and rigid echinus, is everywhere sensitive to stimulation. This sensitiveness, moreover, is highly delicate. If any part of the external surface of an echinus is lightly touched with the point of a needle, all the feet, spines, and pedicellariæ within reach of that part, and even beyond it, immediately close in upon the needle and grasp it tightly. This simultaneous movement of such a little forest of prehensile organs is a very beautiful spectacle to witness. Here we have proof of the function of the pedicellariæ. In climbing perpendicular or inclined surfaces of rock covered with waving sea-weeds, it must be of no small advantage to an echinus to be provided on all sides with a multitude of movable stalks bearing forceps, which can instantly seize a passing frond. The frond being thus arrested, the spines come to the assistance of the pedicellariæ, and both together hold the sea-weed steady till the ambulacral feet have time to establish their hold upon it with their sucking-disks. This operation may be witnessed by drawing a piece of sea-weed over a healthy echinus in the water.
The capability of the spines for co-ordinated action is highly remarkable and interesting. Thus, for instance, if an urchin be taken
out of the water and placed upon a table, it is no longer able to use its feet for walking, as the suckers can act only under water. Yet the animal is able to progress slowly by means of its spines, which are used to prop and push the globe-like shell along in some continuous direction. If a lighted match be held in front of the moving animal, as soon as the echinus comes close enough to feel the heat, all the spines begin to make the creature move away in the opposite direction. There is an urchin-like form of echinoderm called spatangus, which differs from the echinus in having shorter feet and longer spines. When, therefore, a spatangus is inverted it is unable to right itself by means of its short feet, but uses its long spines to perform the manœuvre. The process is a tedious one, and there are generally numerous failures; but the creature perseveres until it eventually succeeds.
- The material and illustrations of this article are drawn from "Jelly-fish, Star-fish and Sea-urchins," by Dr. G. J. Romanes, the latest issue in the International Scientific Series.