Respiration.—The system of water vessels serves the additional purpose of bringing water containing oxygen into contact with various parts of the body, and the starfish was formerly thought to have no special respiratory organs. However there are holes in the aboral wall through which the folds of the delicate lining membrane protrude. These are now supposed to be gills (k, Fig. 58).
The nervous system is so close to the aboral surface that much of it is visible without dissection. Its chief parts are a nerve ring around the mouth, which sends off a branch along each ray. These branches may be seen by separating the rows of tube feet. They end in a pigmented cell at the end of each ray called the eye-spot.
The food of starfish consists of such animals as crabs, snails, and oysters. When the prey is too large to be taken into the mouth, the starfish turns its stomach inside out over the prey (Fig. 59). After the shells separate, the stomach is applied to the soft digestible parts. After the animal is eaten, the stomach is retracted. This odd way of eating is very economical to its digestive powers, for only that part of the food which can be digested and absorbed is taken into the body. Only the lower part of the stomach is wide and extensible. The upper portion (next to the aboral surface) is not so wide. This portion receives the secretion from five pairs of digestive glands, a pair of which is situated in each ray. Jaws and teeth are absent. (Why?) The vent is sometimes wanting. Why?
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Fig. 59.—Starfish eating a sea snail.
b, stomach everted.
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 lighter colored, usually white.[1]
Regeneration after Mutilation.—If a starfish loses one or more rays, they are replaced by growth. Only a very ignorant oyster-*man, angry at the depredations of starfish upon his oyster beds,
- ↑ The sperm cells and egg cells are poured out into the water by the adults, and the sperm cell, which, like all sperm cells, has a vibratory, tail-like flagellum to propel it, reaches and fertilizes the egg cell.
