viii EGHINODEEMATA— NERVOUS SYSTEM 455 



In the Aderoidea and Criitoidea, it occupies throughout life an 

 epithelial position, but in all other Echinodernis it sinks and becomes 

 subepithelial, except at the ends of the radii (in the terminal tentacles), 

 and in the intestine. Here, throughout life, it is epithelial. 



The shifting of the superficial nervous system to a position below 

 the body epithelium, in Echinoderms, goes hand in hand with the 

 formation of the epineural canals, as already explained (p. 449). 



1. Asteroidea (Fig. 354, p. 411). 



The radial nerves here form a thickened longitudinal ridge of the 

 epithelium along the base of each ambulacral furrow, and the circular 

 nerve a thickened ridge round the mouth. In these nerve ridges the 

 (ciliated) epithelial cells represent the nerve cells. At their bases they 

 are continued into nerve fibres, which run in the longitudinal direction 

 of the nerve ridges (i.e. of the radial nerves), and together form the 

 deeper layer of the ridge. From the radial nerves a close plexus of 

 nerve fibres spreads out below the surface of the outer epithelium over 

 the whole body ; this is especially close in the ambulacral feet. In 

 the same way a layer of nerve fibres is found over the whole intestinal 

 epithelium ; this layer increases in thickness orally until it enters the 

 circular nerve. 



2. Crinoidea (Fig. 356, p. 413). 



The above account of the nervous system of the Asteroidea is also 

 applicable to that of the Crinoidea, if it be remembered that the food 

 grooves of the arms and of the tegmen calycis of the Crinoids correspond 

 with the ambulacral furrows of the Asteroidea. The food grooves 

 branch with the arms, and so do the radial nerve ridges of the super- 

 ficial oral system. The Crinoids, however, differ from the Asteroids in 

 that the epithelial nerve plexus is limited to the oral side of the calyx 

 and of the arms, since no epithelium can be made out on the apical 

 side of the calyx, on the sides and backs of the arms, and on the stalk 

 and the cirri of adults. 



3. Ophiupoidea (Fig. 372). 



The superficial oral nervous system, having here taken up a sub- 

 epithelial position, appears in the form of distinct nerve trunks and of 

 nerves radiating from these. The central portion consists of the nerve 

 trunks of the disc and of the five radial trunks of the arms. These 

 latter run on the inner side of the row of ventral shields, between 

 these and the vertebral ossicles. The circular nerve throughout its 

 whole course is beset with nerve cells on the side turned towards the 

 oesophagus, and the same is the case with the radial nerves on the 

 side turned towards the ventral shields. The segmentation of the 

 arms appears in a marked manner in the radial nerve trunks which 



