Chap. 32 ECHINODERMS FORERUNNERS OF THE VERTEBRATES 657 



they are able to work together and the starfish can move in one direction. 

 Progress is slow and often begins only after a period of seeming disagreement 

 among the feet as to which direction they will go. In order to take a step, a tube 

 foot must receive a signal from the nervous system and be stimulated by con- 

 tact with a surface. First the internal bulb (or ampulla) contracts and forces 

 fluid into the external tubular part of the foot which is extended (Fig. 32.4). 

 In the meantime, a valve prevents the fluid from instantly flowing back into 

 the bulb. The extended foot makes a contact with the surface, muscles in the 

 disk or sole of the foot contract, suction is produced, and a foothold estab- 

 lished. Longitudinal muscles in the tube then contract and pull. This is the 

 pull that moves the body of the starfish when many tube feet are working. Fol- 

 lowing this, the longitudinal muscles of the tube relax, and circular ones con- 

 tract and force fluid back into the bulb. The foot is now ready for a refill and 

 another step. Water continually diffuses from the water vascular system into 

 the body cavity and this diffuses through the gills and body wall. With every 

 step some water is lost from each tube foot. 



Nervous System. The central nervous system consists of a nerve ring sur- 

 rounding the mouth and connected with five radial nerves, one in each arm 

 (Fig. 32.6). At the tip of the arm a radial nerve gives off fine branches. This 

 region is highly sensitive to touch and to light through the eyespot. The cen- 



anus 

 rectal sac 



intestine 



sieve plate 



c stomach 



ac stomach 



hollow gastric gland 



Sieve ca 



ossicles 



ring canal'^ nerve ring 



radial 

 nerve cord 



Fig. 32.6. Diagram of a vertical section, tube feet omitted, through the central 

 disk and base of one arm of a starfish, Asterias. A few pedicellariae are shown 

 to indicate their presence; actually gills and pedicellariae are abundant. The 

 cardiac stomach is the part that the starfish extends out through its mouth and 

 spreads over the soft body of a clam or oyster. (After Brown: Selected Inverte- 

 brate Types. New York, John Wiley and Sons, 1950.) 



