364 



ESSENTIALS OF ZOOLOGY 



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- 



Fig. 136. — Dissection of the starfish, Asterias. The aboral wall has been re- 

 moved from the trivium and a portion of the central disc. One ray of the bivium 

 has been turned to expose the oral surface and tube feet. The organs have been 

 removed from one ray of the trivium to expose the skeleton. Avi, ambulacral 

 groove; cs, cardiac stomach; D.B., dermal branchiae; E, eyespot ; G, gonads; M. 

 madreporite ; Os., ossicle ; P, pedicellariae ; P.C., pyloric caeca ; Py, pyloric sac ; 

 R, rectal gland ; Sp., spine ; T.F., tube feet ; T.F.z, arrangement of tube feet in skeletal 

 ray. (From White, Genera? BioZofiry, The C. V. Mosby Company.) 



