ECHINOIDEA. 



229 



the papilliform termination of the radial water-vascular trunk 

 which perforates each of the ocular plates of the periproct. 



The deeper oral system is restricted to five radially-placed 

 patches, close to the circumoral nerve ring, and innervates the 

 muscles of the jaw apparatus. It is absent in Spatangoids. 





17 



FIG. 169. Transverse section through a radial part of the body -wall of an Echinoid (diagram- 

 matic after Delage and Herouard). 1 tubercule for the articulation of 2 a spine ; 3 musculus 

 externus, 4 musculus internus of the base of a spine ; 5 nerve ring at the base of a spine ; 

 6 ambulacral plate ; 7 ectoneural plexus ; 8 pedal nerve ; 9 nerve ring of the sucker, 10 of a 

 tube-foot ; 11 skeletal piece in the pedal sucker ; 12 muscle of foot ; 13 cavity of tube- 

 foot ; 14 ampulla traversed by muscular bands ; 15 canal passing from radial water- 

 vascular canal, 16 to ampulla ; 17 radial blood-strands ; 18 radial perihaemal canal ; 19 

 radial nerve ; 20 epineural canal. 



The apical nervous system appears to be unrepresented, 

 unless an annular nerve trunk in connexion with the aboral 

 circular sinus belongs to it. 



Sense organs. The tube-feet, and pedicellariae and smaller 

 spines are all highly sensitive and provided in the case of the 

 two latter structures with special aggregations of nervous tissue. 

 The terminal tentacle projects slightly on the ocular plate and 



