486 



COMPARATIVE ANATOMY 



ally modified for the purpose of respiration. All these respiratory 

 organs (except the respiratory trees of the Holothurioidea) are described 



Fig. 3S0. — Radial - interradlal section 

 through the disc and the base of the arm 

 of an Ophiurid, in tlie (lin'cti(jn of the jirin- 

 cipal iixis. Lfft liiilf iuten-adial, right lialf 

 radial (after Ludwig). inu, mvi, wk^, Muscles 

 of tlie oral skeleton ; an (black), nerve ring ; 

 f(Hi2+((i/i, oral-angle plates; or, oral = ventral 

 wall of the disc ; aav, apical circular sinus 

 with ring-like strand (c/. Fig. 390). vP, Polian 

 vesicle ; hi, mesenterial filaments between the 

 stomachal sac and the wall of the disc ; 

 cl, ccelom ; aliv, apical wall of the disc ; ra, 

 radial canal of the water vascular system ; 

 rph, radial pseudohaiinal canal ; rv, continua- 

 tion of the axial organ in the arm(?), radial 

 blood vessel (?) ; ni, radial nerve trunk of the 

 oral system : bsi - bsg, first to sixth ventral 

 shield ; feoj and teoo, first and second oral 

 tentacles ; tti, torus angularis ; D, teeth ; 

 0, buccal cavity ; la, entrance to the stomachal 

 sac; iph, peripharyngeal sinus; ami, peri- 

 stomal plates ; ((re, water vascular ring ; 

 it, stomachal sac ; anhi-amg, second to sixth 

 brachial vertebral ossicles ; sjvf, septum be- 

 tween the body cavity and the peripharyngeal 

 sinus. 



in connection with the systems of organs to which they belong, 

 review of these origans will be found below. 



