312 PHYSIOLOGY OF THE INVERTEBRATA. 



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

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

 ambulacral radial canal (Fig. 56, c) ; the lateral branches, 



Fig. 56.— Diagram showing Poktions of Ambulacral and Nervous 



Systems of Echinus. 



{After Romanes and Ewakt.) 



a = ampullae. /- = radial nerve, c = neural radial sinus, d = lining membrane 

 of shell. e = radial ambulacral canal. f — lateral branch of radial canal. 

 ^ = pedicel, /i = spine. 7 = pedicellaria. k = layer of fibres external to shell. 

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

 /" = plexus extending over pedicellaria towards base of mandibles. fn = epider- 

 mis, n — 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 



