68 



ECHINODERMATA. ASTEROIDEA. 



water-vascular system is placed in communication with 

 the sea- water by means of a canal (fig. 16 d) passing from 

 the circular vessel to the dorsal surface of the disc and 



FIG. 16. Diagram of the water- vascular system of a starfish, a, circular 

 vessel round the mouth ; b, radial vessels ; c, Polian vesicles ; 

 d, stone-canal; e, madreporic plate; /, tube-feet (only a few shown); 

 <7, ampulla. 



ending in the madreporic tubercle. This is known as the 

 stone-canal on account of the deposit of carbonate of lime 

 in its walls. 



The distribution of the nervous system is similar to 

 that of the water-vascular system. It consists of a ring 

 round the mouth from which a branch goes down the 

 ambulacral groove of each arm, being situated immediately 

 under the epidermis, and external to the water-vascular 

 vessel ; each radial nerve terminates at the end of the ray 

 in an eye. The blood vascular system consists of a dorsal 

 and a ventral ring, one round the mouth and the other 

 round the anus, from each of which radial branches are 



