Phylum Echinodermata and Related Groups 547 



The mouth opens into a narrow esopJmgus, which in turn opens 

 into the expanded stomach. The stomach has two portions : the sac- 

 hke cardiac and the narrower pyloric which are separated from one 

 another by a constriction. From each arm, a pair of large hepatic 

 caeca which secrete powerful proteolytic enzymes open into the pyloric 

 portion of the stomach. Extending toward the aboral surface from the 

 pyloric stomach is the short intestine which terminates in the anus. 

 Opening into the intestine is the pair of rectal caeca. 



MAOREPORlTE 



Fig. 180. — Anatomy of the starfish, Asterias. 



Between the rows of tube feet in a single arm is located a radial 

 nerve. These fave nerves join the nerve ring in the mouth area. To- 

 gether these form the central nervous system. Associated with this 

 system is a complex nerve plexus which is under the whole epithelium. 

 Other smaller nerves supply the tube feet and ampullae while the der- 

 mal papillae, spines, and pedicellariae are supplied by the epithelial 

 plexus. At the tip of each arm is a light-sensory area, the eyespot. 



The sexes are separate, with the gonads lying in each arm and 

 opening to the aboral surface by a very small duct. 



The Life History of A. forbesi. — -Ripe gonads can be found in 

 this species during the spring and early summer months. The eggs 

 and sperm are shed directly into the water and fertilization and develop- 

 ment follow. Cleavage is regular and holoblastic, proceeding through 

 the typical blastula, gastrula, and mesoderm formation. As the ani- 

 mal continues its development, ciliated bands develop on the outer 

 surface and serve for locomotion. This larva gradually develops three 

 lobes on each side of the body, producing the hipinnaria larval stage 



