486 



COM PA RA TI VE ANA TOMY 



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). m //, //< // x , mil a, Muscles 

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

 r</._,+(7j, oral-angle plates; or, oral = ventral 

 wall of the disc; aav, apical circular sinus 

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

 vesicle ; lii, niesenterial filaments between the 

 stomachal sac and the wall of the disc ; 

 d, ccelom ; ulw, apical wall of the disc ; m, 

 radial canal of the water vascular system ; 

 rjih, radial psi-udohivmal canal ; IT, continua- 

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

 blood vessel(?); rn, radial nerve trunk ot'tlin 

 oral system: bsj-bsg, first to sixth ventral 

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

 tentacles ; /</, torus angnlaris ; L>, teeth ; 

 o, buccal cavity ; la, entrance to the stomachal 

 sac; f'yi/i, peripharyngeal sinus; ii, peri 

 stomal plates ; nn, water vascular ring ; 

 it, stomachal sac ; ;/(_> amg, second to sixth 

 brachial vertebral ossicles ; spt, septum be- 

 tween the body cavity and the peripharyngeal 

 sinus. 



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

 review of these orerans will be found below. 



