486 



COMPARATIVE ANATOMY 



CHAP. 



ally modified for the purpose of respiration. All these respiratory 

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



I ' i - ; . 3SG. 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 halt' 

 radial (after Ludwlg). mn, niu } , nui.,, Muscles 

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

 anio+ff/i, oral-angle plates; or, oral = ventral 

 wall of the disc; u.av, apical circular sinus 

 with ring-like strand (</. Fig. 300). vP, Polian 

 vehicle ; /,/. nieseiiterial tilanients between the 

 stomachal sac and the wall of the disc; 

 '/, cceloin ; (</!'-, apical wall of the disc; ra, 

 radial canal of the water vascular system ; 

 rph, radial pseudoha-mal canal ; rv, continua- 

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

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

 oral system: /,>,-//>,., first to sixth ventral 

 shield ; teoi and t>. 2 , tirst and second oral 

 tentacles ; /", torus an^ularis ; /', teeth ; 

 o, buccal cavity ; la, entrance to the stomachal 

 sac; iph, peripharyngeal sinus; cm^ peri- 

 stomal plates ; nn, water vascular ring ; 

 it, stomachal sac ; n 1,1-2 -a?ng> second to sixtli 

 brachial vertebral ossicles ; spt, septum be- 

 tween the body cavity and the peripharyngeal 



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

 review of these organs will be found below. 



