174 INVERTEBRATE ZOOLOGY 



The nervous system consists of a circumoral nerve ring with 

 a radial nerve extending along each ambulacral area. The 

 system is peculiar in that it retains a superficial location in the 

 ectoderm. Minute branches are given off to the various organs. 

 There are no centralized ganglia in this system but experiments 

 indicate that the nerve ring serves as a coordination center. 



Musculature.— Though there are no conspicuous muscle 

 bundles encountered in the dissection of a starfish, muscle tissues 

 play exceedingly important roles in the structure of this animal. 

 The walls of the tube-feet are largely composed of muscle. 

 Minute muscles manipulate the pedicellariae. Spines are moved, 

 and the skeletal plates are articulated, by muscles. The stomach 

 is everted by muscular action, and special bundles of muscles 

 retract it. Movements of the arms are produced by delicate 

 sheets of muscles just beneath the dorsal wall of each ray. 



The common starfishes belong to the genus Asterias. Cteno- 

 discus has a pentagonal body without conspicuous arms. Solas- 

 ter and Pycnopodia have numerous arms. Henricia is a northern 

 genus with only two rows of feet on each ray. 



Class Ophiuroidea 



The brittle stars or serpent stars have highly flexible arms 

 radiating from a central circular or pentagonal disc. Though 

 superficially resembhng the asteroids, they differ radically from 

 them in details of organization. The ambulacral system is 

 much reduced and fails to function in locomotion but serves 

 rather as a series of tactile organs. Writhing movements of the 

 arms produce locomotion. The ambulacral plates are withdrawn 

 into the interior of each ray where they are fused together to 

 form a jointed rodlike structure the units of which are called the 

 vertebrae. Both internally and externally, each arm is composed 

 of a large number of similar segments. In the basket stars, the 

 arms become finely branched. The digestive system is confined 

 to the disc and lacks an anus. Bursae are thin-walled sacs, 

 leading inward from the ventral surface of the disc, which serve 

 for respiration and into which the gonads open. 



On the oral surface of the disc, five interradial groups of plates 

 project in toward the mouth to form the jaws which are operated 

 by muscles for masticating food or for selecting food particles. 

 At the base of each jaw there are usually three plates, a large 



