ECHIXODERMS {SPINY ANIMALS) 



37 



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 mem- 

 brane 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 a 



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 ? 



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, 



1 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,_c«aches and fertilizes the egg cell. 



Fig. 59. — Starfish eat- 

 ing a sea snail. 

 b, stomach everted. 



