408 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



away from injuries inflicted on them. If the nerve 

 ring be divided, without the separation of a ray from 

 the rest of the organism, the ray whose nervous con- 

 nection is so cut ceases to act with the rest of the star- 

 fishj-hiit. is fifjpa.hlft, f.r> q, pfivhn.in extent, of responding 

 tO ^IIP" 11 ' 9" ifa 17W 1 ! 



Fig. 176. Separate Segment of an Echinus attempting to right itself 

 - after having been inverted. (After Koinanes and Ewart.) 



The Criiioidea must be dealt with separately from 

 the rest of the Echinodermata, in consequence of the 

 difficulties presented by the conditions and relations 

 of their nervous system. When a transverse section, 

 is made of one of the pinnules which, hang down from 

 the sides of an arm of a Crinoid, a nerve cord (n ; Fig. 

 177) is seen to lie just underneath the epithelium of 

 the groove of the pinnule ; this clearly corresponds to 



