356 



COMPARATIVE ANATOMY 



regular segmental intervals through apertures Avhich lie on each side 

 between the ventral shields and the lateral shields belonging to them 



cfj> c<J 



"-JiP 



3_5 77^./"^^ 



Fig. 311.— Transverse section through the arm of an Ophiurid (after Ludwig). Uiagiaiu. 

 «s, Lateral sliieUls ; ch, dorsal shields ; (/, cavity of tlie arm (cn-loin) ; ac, spines ; am, the anibu- 

 lacral plates (vertebrse) ; .r, loop of tentacle canal in the groove on the distal face of the ossicle (cf. 

 next fig., A 4) ; vie, tentacular canal of the radial vessel (w) of the water vascular system ; t,\ 

 feeler (tentacle) ; re, radial pseudoliEenial vessel ; rn, radial nerve strand ; hs, ventral shield. 



Fig. 312.— Vertebral ossicles (ambulacral plates) of Ophiarachna incrassata (after Ludwig), 

 to show the articulating prominences and depressions, etc. A, Tliree vertebral ossicles from tlie 

 side. B, Vertebral ossicles from the proximal (adoral), and C, from the distal (aboral) side. D, Three 

 vertebral ossicles from the ventral side, pr, Proximal ; di, distal ; ra, radial trunks of the water 

 vascular system ; rn, radial nerve trunk ; rv, radial pseudohaemal canal. 1, Point at whicli the 

 branch of the radial water vascular trunk running to the tube-foot passes out of the substance 

 of the vertebral ossicle at its distal side ; 2, point where this branch re-enters the ossicle ; 

 4 channel between these two points, which receives the loop of the branch belonging to the 

 tube-foot • 3 depression for the lower intervertebral muscle ; 5, channel for the radial W'ater 

 vascular trunk ; 6, depression for the tube-foot ; 7, channel for the branch of the nerve running to 

 the tube-foot ; 8, pseudoliaemal vessel to the same ; 9, ner\-e branch to the same ; 10, brand) of the 

 water vascular system to the same, which at 12 jjasses into the substance of the ossicle, and at 13 

 out of the latter and into the tube-foot; 11, point at whicli tlie nerve branch (14) running to the 

 upper intervertebral muscle, enters the vertebral ossicle. 



((/. Fig. 245, p. 300). At the edge of these apertures there are smaller 

 spines or scales. 



The axial double plates are called vertebral ossicles, a very suit- 



