XVI 



ASTERIAS ALIMENTARY CANAL — FOOD 



439 



and attached to the dorsal wall of the coelom by suspensory 

 bands of membrane called mesenteries. These ten forks are called 

 " pyloric caeca " ; they are of a deep green colour owing to the 

 pigment in their wall. Beyond the pyloric sac the alimentary 

 canal is continued as the slender " rectum " to the anus. The 

 rectum gives off two small branched pouches of a brown colour 

 called " rectal caeca." This comparatively complicated form of 

 alimentary canal is related to the- nature of the food of the animal 

 and the method it employs to capture its prey. 



The favourite food^ of Asterias consists of the common bivalves 



^k\i^l 



Fjg. 1S9. — View of a Starfish (Echinaster) devouring a Mussel. 1. The madreporite. 



of the coast, notably of the Mussel (Mytilus edulis). There is, 

 however, no animal which it will not attack if it is fortunate 

 enough to be able to catch it. The Starfish seizes its prey by 

 the tube-feet, and places it directly under its mouth, folding its 

 arms down over it in umbrella fashion. The muscles which run 

 around the arms and disc in the body-wall contract, and the 

 pressure thus brought to bear on the incompressible fluid con- 

 tained in the coelom, forces out the thin membranous peristome 

 and partially turns the stomach inside out. The everted edge of 

 the stomach is wrapped round the prey. 



1 starfish are most destructive on oyster-beds, and hence possess considerable 

 negative economic value. 



