486 



COMPARATIVE ANATOMY 



CHAP. 



ally modified for the purpose of respiration. All these respiratory 

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



Fig. 380. — Radial - interradial section 

 through the disc and the base of the arm 

 of an Ophiurid, in the direction of the prin- 

 cipal axis. Left half interradial, right half 

 radial (after Ludwig). mu, m«i, ^11(2, Muscles 

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

 am-2-hadi, oral-angle plates; or, oral=ventral 

 wall of the disc ; aav, apical circular sinus 

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

 vesicle ; hi, mesenterial filaments between the 

 stomachal sac and the wall of the disc ; 

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

 radial canal of the water vascular system ; 

 rpli, radial pseudohseinal canal ; rv, continua- 

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

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

 oral system : hsi - bsQ, first to sixth ventral 

 shield ; teoj and teo2, first and second oral 

 tentacles ; tn, toiiis angularis ; D, teeth ; 

 0, buccal cavity ; la, entrance to the stomachal 

 sac ; iph, peripharyngeal sinus ; ami, peri- 

 stomal plates ; art, water vascular ring ; 

 it, stomachal sac ; aiii^-aviQ, second to sixth 

 brachial vertebral ossicles ; spf, septum be- 

 tween the body cavity and the peripharyngeal 

 sinus. 



in connection with the systems of organs to which they belong 

 review of these oreans will be found below. 



