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.
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.