STARFISH AND OTHER ECHINODERMS 



365 



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

 constriction. A large pair of branched glandule structures, known 

 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 

 over it. The tube feet are attached to the shells, and, by coopera- 

 tive activity, an enormous pull is exerted on the valves of the 

 shell. After the shell is open, the stomach of the starfish is everted 



Fig 137. — Longitudinal section through the central disc and one ray of a star- 

 fish, a, Anus ; am, ampulla ; car, cardiac stomach ; coe, perivisceral coelome ; ey, 

 eyespot ; hca, hepatic caeca ; i, intestine ; m, mouth, mth madreporic plate ; nr, 

 nerve ring ; oe, esophagus ; os, ambulacral ossicle ; Py, pyloric sac ; 7'a, radial canal ; 

 re, ring canal ; rca, rectal caeca ; rn, radial nerve ; Sc, stone canal ; sp, spme ; tf, 

 tube feet. (From W^hite. General Biology, The C V. Mosby Company.) 



through its mouth and is spread over the tissues of the prey. An 

 abundance of digestive fluid secreted over the food causes it to be 

 digested in its own shell, and it is then taken into the stomach of the 

 starfish. It is reported that between four and five dozen clams may 

 be eaten by a single starfish in a week. It has also been shown 

 that a starfish may survive after months of fasting. After feed- 

 ing, the stomach is withdrawn into the body cavity by five pairs 

 of retractor muscles, one pair extending from the pyloric portion to 

 the ambulacral skeleton of each arm. The branched, treelike gonads 

 fill the remaining space in each arm and the external pores from 

 them are located in the crevice between adjacent arms. 



The water-vascular system is composed of the madreporite, stone 

 canal, circumoral or ring canal, radial canals, Tiedemann's bodies, 

 lateral canals, ampullae, and tube feet. Water is taken- in through 



