The Sea Shore

by William S. Furneaux

The various species of Brittle Stars live among the rocks and weeds, chiefly in deep water, where they move about by means of the muscular contraction of their arms, the disc being raised on the curved arms as the animal proceeds. Some species are to be found between the tide-marks, and especially abundant on the south-west coast are two small species that live among the tufts of coralline weeds, sometimes so crowded together that dozens may be taken from a little patch of coralline only two or three inches square. These have such small discs, and such slender arms, and are, moreover, so well concealed by their colouring, which closely resembles that of the weed-tuft they inhabit, that they are only to be detected by close inspection.

The remaining division of the starfishes, sometimes distinguished by the name of Common Stars, possess five arms or rays, which may be either long or short, and which are not jointed with the central disc, but continuous with it; that is, there is no sharp line of demarcation between arm and disc. One or two species are well known to all frequenters of the sea-side, but the majority of them inhabit deep water, where they creep about over the rocks and weeds, obtaining their food from the bed below them.

If we examine the common five-finger star that is so often stranded on the beach, and so frequently found in rock pools between the tide-marks, we see that each arm has a large and conspicuous groove running along its centre on the under side, and on each side of these are the rows of tube-feet, arranged in such a manner that they have suggested the appearance of an avenue of trees on each side of a garden walk, and have consequently earned the name of ambulacrum. These tube-feet may be protruded for some distance; and, being provided with suckers that possess considerable clinging power, they form the principal means of locomotion.

Put the starfish in the aquarium, or in a tidepool by the sea, and you will find it very interesting to observe how the animal progresses, while some idea of the clinging power of the tube-feet may be ascertained by allowing the animal to creep over the submerged hand.

The movements of the tube-feet may also be seen to advantage when the starfish is laid upside down in a pool, and, what is still more interesting, the manner in which the animal turns itself over. To do this it will first bend the tips of one or two of its arms so as to bring the suckers against the ground; and then, aided by the pulling action of these, it will gradually bring other suckers into a similar position till, at last, the whole body has been turned over. Some of our common starfishes have rays so short that they may be termed angles rather than arms, and these are unable to turn their inverted bodies by the gradual method just described. They generally raise their bodies on the tips of three or four of the rays, assuming somewhat the form of a three-or four-legged stool, and then, bending the remaining one or two arms over the body, they alter the position of the centre of gravity till eventually the body topples over to the desired position.

Some of the common five-rayed stars have no suckers on their tube-feet, and consequently have to creep by means of the muscular contractions of their arms; and several of them are like the brittle stars in breaking up their bodies when irritated or seized. This latter peculiarity will account for the frequency with which we come across animals with one or more rays smaller than the others, the smaller rays being new ones that have been produced in the place of those lost. Again, we sometimes meet with such monstrosities as a five-rayed star with six or more rays, some smaller than others, the smaller ones representing two or more that have grown in the place of one that has been lost; or a starfish with branched or forked arm, illustrating the tendency to produce a new arm even when the original one has been only partially severed.

A close observation of a starfish in water may enable us to detect a number of little transparent processes standing out between the prominences of the rough skin of the upper surface. These are little bags filled with fluid, formed of such thin walls that gases can readily pass through them, and are undoubtedly connected with the process of respiration. Also, on the upturned extremity of each arm a red spot may be seen; and this from the nature of its structure, and from its association with the nervous system, has been regarded as a rudimentary eye.

On the upper side of the disc one may also observe a more or less conspicuous spot of variable colour, on one side of the centre. It is a plate, finely perforated, covering the outer extremity of a short canal which communicates with the system of water tubes that were described in the earlier part of this chapter. It is, in fact, the entrance through which water is admitted into the central ring round the mouth, and from this into the radial water tubes that run through each arm of the starfish to supply the tube-feet. The short tube referred to is always filled with sand, and thus the water that enters into the water-vascular system is filtered before it reaches the circular vessel. It is interesting to note, in this connection, that here is one respect in which the radiate symmetry of the starfish is broken, there being only one entrance, and that not a central one, by which water is distributed into the five rays.

Of course, when the ray of a starfish has been broken off the water vessel or vessels that it contained are destroyed, as is also the prolongation of the stomach, in the form of a long, blind tube, that extended into it. But no inconvenience attaches itself to this loss, for the starfish has the power of reproducing even its lost viscera, as well as any of the five rays of the body that may be broken off.

We must briefly refer to one other feature of the common star, viz. the possession of small prehensile organs around the mouth. These are little spines, the extremities of which are movable, and take the form of little pincers by means of which the animal can hold its prey.