124 



PHYLUM ECHINODERMATA. 



opening (perioesophageal nerve ring), is found throughout the 

 endoderm and is apparently continuous at the mouth and 

 possibly at the anus with the ectoneural plexus. 



In Echinoids, Holothurians and Ophiuroids, the circumoral 

 ring and radial nerve trunks are separate from the superficial 

 ectoderm and placed in the wall of a canal, the epineural canal 

 (Figs. 140, 169, 180), which is actually formed in development 

 by the invagination of the larval ectoderm along the centre of 

 each radius* (p. 150). So that in these forms the ectodermal 

 part of the central nervous system attains its internal position 

 by invagination as it does in Enteropneusta and Verte- 



Flo. 86. Scheme of the nervous system of the arm of a starfish (after Cuenotl. a wall; b 

 body-cavity of arm ; c ampulla of tube foot ; rftube foot ; e radial canal of water- vascular 

 system ; 1 radial portion of ectoneural central nervous system ; 2 ectoneural plexus of 

 tube-foot ; 3 ectoneural plexus of skin ; J Lange's nerve cords (deep oral) ; 5 mesoneural 

 plexus just beneath the longitudinal muscle. 



brata, and the epineural canal may be compared to the 

 central canal of the nervous system of those animals. Special 

 nerve trunks pass from the central parts of this system to the 

 skin, tube-feet, etc. At the end of the radii these radial trunks 

 pass to the surface and lie in the ectoderm covering the terminal 

 tentacle, if such is present. In the same three classes the apical 

 nervous system (see below) is not present, or at any rate not 

 developed in the same marked manner that it is in Crinoids and 

 Asteroids. 



The deep oral nervous system consists of a double cord in each 



* This has been shown for Echinoids by Mac-Bride, for Ophiuroids by 

 Grave, and for Holothurians by Clark. 



