Reproduction.—There is a pair of ovaries at the base of each ray of the female starfish (Fig. [58]). The spermaries of the male have the same position and form as the ovaries, but they are of a lighter colour, usually white.[[2]]
[2]. The sperm cells and egg cells are poured out into the water by the adults, and the sperm cell, which, like nearly all sperm cells, has a vibratory, taillike flagellum to propel it, reaches and fertilizes the egg cell.
Regeneration after Mutilation.—If a starfish loses one or more rays, they are replaced by growth. Only a very ignorant oysterman, angry at the depredations of starfish upon his oyster beds, would chop starfish to pieces, as this only serves to multiply them. This power simulates multiplication by division in the simplest animals.
Fig. 60.—Young starfish crawling upon their mother. (Challenger Reports.)
Steps in Advance of Lower Branches.—The starfish and other echinodermata have a more developed nervous system, sensory organs, and digestion, than forms previously studied; most distinctive of all, they have a body cavity distinct from the food cavity. The digestive glands, reproductive glands, and the fluid which serves imperfectly for blood, are in the body cavity. There is no heart or blood vessels. The motions of the stomach and the bending of the rays give motion to this fluid in the body cavity. It cannot be called blood, but it contains white blood corpuscles.
The starfish when first hatched is an actively swimming bilateral animal, but it soon becomes starlike (Fig. [60]). The limy plates of the starfish belong neither to the outer nor to the inner layer (endoderm and ectoderm) of the body wall, but to a third or middle layer (mesoderm); for echinoderms, like the polyps, belong to the three-layered animals. In this its skeleton differs from the shell of a crawfish, which is formed by the hardening of the skin itself.
Protective Coloration.—Many starfish are brown or yellow. This makes them inconspicuous on the brown rocks or yellow sand. Brightly coloured species are usually chosen for aquaria.
The Sea Urchin
External Features.—What is the shape of the body? What kind of symmetry has it? Do you find the oral (or mouth) surface? The aboral surface? Where is the body flattened? What is the shape of the spines? What is their use? How are the tube feet arranged? Where do the rows begin and end? Would you think that a sea urchin placed upside down in water, could right itself less or more readily than a starfish? What advantage in turning would each have that the other would not have? The name sea urchin has no reference to a mischievous boy, but means sea hedgehog (French oursin, hedgehog), the name being suggested by its spines.