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, mu\, mil*, Muscles 

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

 am-2+adi, oral-angle plates; or, oral = ventral 

 wall of the disc; aav, apical circular sinus 

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

 vesicle ; In, mesenterial filaments between the 

 stomachal sac and the wall of the disc ; 

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

 radial canal of the water vascular system ; 

 rph, radial pseudohsemal canal ; n; continua- 

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

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

 oral system: bsi-bs^, first to sixth ventral 

 shield ; teo\ and teo->, first and second oral 

 tentacles ; tn, torus angularis ; D, teeth ; 

 o, buccal cavity ; la, entrance to the stomachal 

 sac; iph, peripharyngeal sinus; o.nij, peri-" 

 stomal plates ; aa-, water vascular ring ; 

 it, stomachal sac ; am.? - am^, 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. 



