52 Invertebrate. 



pedicellaria (' little feet ') is given. From the central 

 stomach, which communicates directly with the mouth, 

 long blind pouches extend into the arms (usually two 

 pouches into each arm) thus increasing the size of the 

 digestive sac. There is an ambulacral ring around 

 the mouth, and radial vessels extend from it into the 

 arms to supply the little feet ; and to convey sea water 

 into the central ring there is a canal, usually filled 

 with sand, which starts on the dorsal surface of the 

 disk from a spot where there is a wart-like, finely 

 perforated plate, which from its likeness to a piece of 

 coral is called the madreporiform plate. Through it, 

 as through a sieve, the sea-water filters into the sand- 

 canal, and thence into the ring around the mouth. 



Besides the ambulacral vessels, there exist in star- 

 fishes fine vessels on the surface of the digestive 

 cavity, which unite to form a second vascular ring 

 around the mouth ; this second system is one of blood- 

 vessels directly concerned in the nutrition of the body. 

 The larvae of starfishes, on leaving the egg, appear 

 very dissimilar from the adults, looking like little 

 easels, and are hence called plutei (fig. 30) ; those of the 

 brittle-stars have a delicate calcareous skeleton, which 

 is wanting in those of the common five-fingered 

 starfishes, which in many respects seem to resemble 

 the comb-bearing Hydrozoa. In their adult states these 

 starfishes move mouth-downwards and are extremely 

 voracious, attacking molluscs, dead fishes, and other 

 kinds of animal matter in the sea. 



