PHYLUM ECHINODERMATA 



229 



are shorter and are known as adamhulacral plates. Five flat oral 

 ossicles surround the mouth. 



Within the body wall and extending into the arms is a large eoelom 

 which is lined by a pcrito7ieum and filled with coelomic fluid. In 

 this cavity are located the organs of most of the systems. The 

 digestive system is a modified tube extending vertically from the 

 mouth on the oral side to the minute arms at the aboral surface. 

 From the mouth a short esophagus leads to the double-pouched 

 stomach. The larger cardiac portion (or pouch) receives the esopha- 

 gus and is separated aborally from the pyloric portion by a marked 

 constriction. A large pair of branched glandular structures, known 



Fig. 124. — Diagram of a cross section through a ray of a starfish, avi, ampulla ; 

 amb, ambulacral ossicle ; coe, perivisceral coolom ; db, dermal branchia ; hca, 

 hepatic caeca ; musj muscle ; oSj ossicle ; pd, pedicellaria ; ph, perihemal space ; ra, 

 radial canal ; rn, radial nerve ; rv, radial blood vessel ; sp, spine ; Sp, septum m 

 radial blood vessel (rv) ; tf, tube feet; v, valve between tube foot and radial 

 canal. (From White, General Biology.) 



as hepatic or pyloric caeca, is located in each arm, and each pair 

 joins the pyloric pouch by a duct which seems to be a continuation 

 of this pouch. These glands and possibly the pyloric pouch pro- 

 duce digestive enzymes in solution. The fluid secreted by the wall 

 of the cardiac portion probably does not contain enzymes. A short 

 rectum or intestine leads aborally from the pyloric pouch to the pore- 

 like anus at the exterior surface of the central disc. Two brown, 

 branched pouches arise from the rectum. These are known as rectal 

 caeca or glands and probably have excretory function. In feeding, 

 the starfish catches its bivalve prey in the five arms and humps 



