THE PHYLA HEMICHORDATA AND ECHINODERMATA 369 



Stomach to the ventral body wall of the rays contract to pull the cardiac 

 stomach back inside the disc. 



Aster ias feeds mostly on live bivalves whose shells close tightly. The 

 starfish can open these easily in a few minutes, although the mechanism 

 is not entirely understood. Apparently the starfish grips the two shells 

 with its many tube feet and pulls slowly and steadily. As soon as the 

 bivalve opens the least bit the cardiac stomach is slipped inside and 

 digestion begins. Some of the larger starfish, such as the genus Pisaster 

 of the west coast, are so powerful that they will break the shells of 

 bivalves that are wired shut. 



The nervous system of Asterias is composed of a nerve ring en- 

 circling the mouth and five radial nerves adjacent to the lower epidermis 

 (Fig. 19.1). Other fibers have been identified in the walls of the digestive 

 tract and inside the upper body wall. The separated rays of a starfish 

 each with a pie-shaped piece of the disc will continue to creep for a few 

 minutes in the same direction as they were creeping before being sep- 

 arated. After a few minutes, however, all of the rays will creep with 

 the tip forward as though all of them were acting as anterior rays. These 

 pieces retain a part of the nerve ring and its junction with the radial 

 nerve. If the radial nerve is severed at its junction with the nerve ring, 

 then the ray will creep with its base forward, as though it were a pos- 

 terior ray. This suggests that the part of the nerve ring near each radial 

 nerve is a center from which stimuli pass along the arm and cause it to 

 advance with the tip forward. In the intact animal the centers on one 

 side temporarily inhibit those on the other, permitting the animal to 

 move in a coordinated manner in one direction. 



The large body cavity of Asterias surrounds all the digestive organs 

 and extends to the tips of the rays. Although the coelom appears in the 

 embryo as a pair of lateral cavities, these migrate and come to lie one 

 above the other after metamorphosis. In Asterias the horizontal mesen- 

 tery dividing this pair of cavities disappears except for the five pairs of 

 retractor muscles of the cardiac stomach. 



All over the top and sides of the starfish the body cavity projects 

 through the body wall as numerous tiny papillae covered with epidermis. 

 The ciliated epithelium lining the body cavity circulates the coelomic 

 fluid rapidly in and out of these papillae; they probably function in 

 respiration. The coelomic fluid contains numerous wandering cells that 

 gather up waste. When carmine particles are injected into the body 

 cavity they are picked up by these cells. After a few minutes the cells can 

 be seen in the papillae, and many of them leave the body cavity by 

 crau'ling through the wall to the outside, thus removing the carmine 

 from the body. Whether this is a usual or major method of eliminating 

 wastes is not known. 



The circulatory system of Asterias is composed of circular and 

 radial vessels filled with a fluid similar to that of the body cavity, which 

 in turn is not very different from sea water. The vessels lie above the 

 nervous system enclosed in a body cavity of their own, derived em- 

 bryologically from a part of the coelom. Contractions have been ob- 

 served in some of the vessels. 



