The Childhood of the Starfish and the Sea Urchin

by W. P. Pycraft, F. Z. S., A. L. S.

There are probably not many of my readers who cannot tell a starfish or a sea-urchin at sight, that is to say, a grown-up starfish or urchin; but to distinguish between them, or even to recognise them at all, in the days of their infancy is a very different matter. Indeed, only those who devote their lives to the study of these creatures are able to do this, and the facts which their labours have brought to light are curious indeed, though so complex that it would be impossible to describe them here in full detail.

Fig. 1.—Young Starfish. Fig. 1.—Young Starfish.

An outline, however, of what we may call the story of the starfish can be told readily enough, and without in any way losing aught either of its importance or its interest.

Fig. 2.—Young Starfish, second stage. Fig. 2.—Young Starfish, second stage.
Fig. 3.—Young Starfish, third stage. Fig. 3.—Young Starfish, third stage.

Briefly, among the starfish people—and including also the sea-urchins and sea-cucumbers, the curious brittle-stars and feather-stars—parental care is the exception, and not the rule. Having cast their eggs adrift upon the sea, the mothers of the families leave the rest to nature. Let us follow the history of one of these eggs. No sooner is it adrift than it begins a very remarkable career. Starting at first as a tiny ball, it divides next into two precisely similar balls, and since these divide again and again in like manner, we have in a few hours a mass of little balls, intimately connected with one another, and resembling a mulberry in appearance, enclosing a hollow space. (Fig. 1.)

This stage reached, the end of the first chapter in the life of the starfish is closed. He has grown so far, it should be noticed, without eating; but for further progress food is necessary. Now, this food cannot be taken in without a mouth and some sort of stomach. These are formed by the simple device of tucking in one side of the ball, just as one might push in one side of an indiarubber ball; the rim of the hollow thus formed becomes the mouth, and the hollow into which it leads is the stomach, while within the space lying between the outer wall and that portion of the wall which is pushed in—which corresponds to the inside of the indiarubber ball—the body that is to be begins to be formed. To grasp this thoroughly, first of all take such a rubber ball as I have described, and push in one side. Compare it with the illustration (fig. 2).

Soon after the formation of the mouth, our growing starfish develops his first organs of locomotion. Now, these are neither arms nor legs, but take the form of short hair-like growths, endowed with the power of rapid waving motion, whereby the body is propelled through the water. These are to be seen in the picture of one of these little creatures, shown for clearness sake as if cut in half. (Fig. 3.)

Fig. 4.—Nearly full-grown Starfish. Fig. 4.—Nearly full-grown Starfish.

A little later our young starfish has assumed a new shape. Here there is a large mouth and stomach, while the swimming hairs have all been cast off except a few arranged in the form of two bands; and, later still, the creature takes the extraordinary form shown in fig. 4. Swimming by the motion of waving hairs is now a thing of the past; instead, long arms have been developed, which perform this work much more effectually, and these arms are supported by a hard, chalky skeleton. Soon another little pushing in of the body takes place, and, lo, out of this grows the body of the starfish that is to be! (as is the middle of fig. 4). In about forty-five days from the beginning of this eventful history, the feet and body appear sticking out of the body, whose growth we have been watching; and, in a very short time after, this chalky skeleton is destroyed, and the rest of this infantile body cast away, leaving the fully formed starfish with an entirely new skeleton! Thus, then, wonder of wonders, this curious creature possesses during its lifetime two distinct skeletons!

Fig. 5.—Young Sea-urchin. Fig. 5.—Young Sea-urchin.
Fig. 6.-Young Rosy Feather-star. Fig. 6.-Young Rosy Feather-star.
Fig. 7.—Rosy Feather-star. Fig. 7.—Rosy Feather-star.

The sea-urchin and sea-cucumber undergo similar changes. (See fig. 5.) So also does the beautiful rosy feather-star, but with certain modifications too interesting to be passed over. In what we call its larval body, or its period of childhood, the body takes the form of a cylinder, as you see in the picture, with a little tuft of swimming hairs at the bottom, and bands of the same round the body (fig. 6.) Within this body, as in the starfish, a new body is gradually formed. Then, as you see in the picture, the inside of the egg-shaped body takes the form of a long stalk of stony plate, surmounted by a number of square plates pierced with holes, and these last only are destined to survive in the body of the adult. Soon after this stage is reached, the swimming body comes to rest, because the stalked body which it contains has reached its full development, and takes over the threads of life. As a consequence, the barrel-shaped swimming body, now useless, is thrown off, much as a caterpillar throws off its skin, leaving the newly fashioned body, shaped like a filbert-nut, but rounder, fixed by its stalk to the ground. In a very little while, however, it puts forth a number of beautiful moving arms. It is now a sea-lily! And now follows another change. Breaking away from the traditions of its tribe—the sea-lilies—it cuts itself off from the stalk, and grows in its place a number of short finger-like processes, and lo! from a sea-lily it becomes the rosy feather-star! (What this looks like you can see in fig. 7.) Once more it is able to swim, and this is done by waving movements of the long arms. When desirous of rest, it drops to the bottom of the sea, and clutches hold of some bit of rock or branch of seaweed by means of the bunch of 'fingers' below the body, which we have just described.