312 



PHYSIOLOGY OF THE INVERTEBRATA. 



plates. Each trunk lies in a siuus (Fig. 56, c) situated 

 between the lining membrane of the shell (Fig. 56. cZ) and the 

 ambulaoral radial canal (Fig. 56, e); the lateral branches, 



Fig; 56. — DiAGEAM showing Portions of Ambulackal and Nervous 

 Systems of Echinus. 



[After Romanes and Ewaht.) 



a = ampullae, b = radial nerve, c = neural radial sinus, d = lining membrane 

 of shell. e = radial ambulacral canal. / = lateral branch of radial canal. 

 ,f = pedicel, h = spine, i = pedicellaria. k = layer of fibres external to shell. 

 / = subepidermic nerve-plexus. /' = plexus extending over base of spine. 

 I" = plexus extending over pedicellaria towards base of mandibles. m = epider- 

 mis. 71 = lateral branch from nerve-trunk, o = continuation of lateral branch. 

 ^ = portion of lateral branch, r = ambulacral plate. 



which accompany the first series of pedicels through the oral 

 floor are large and deeply pigmented; the other branches 

 within the auricles are small ; those external to the auricles 



