356 



COMPARATIVE ANATOMY 



regular segmental intervals through apertures which lie on each side 

 between the ventral shields and the lateral shields belonging to them 



."-* 



^ s 771 /*# 



FIG. 311. Transverse section through the arm of an Ophiurid (after Ludwig). Diagram. 

 ss Lateral shields ; ds, dorsal shields ; cl, cavity of the arm (ccelora) ; oc, spines ; am, the ambu- 

 lacral plates (vertebrae) ; x, loop of tentacle canal in the groove on the distal face of the ossicle (c/. 

 next tig., A 4) ; rte, tentacular canal of the radial vessel (ra) of the water vascular system ; te, 

 feeler (tentacle) ; re, radial pseudoluemal vessel ; rn, radial nerve strand ; bs, ventral shield. 



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

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

 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 pseudohsemal canal. 1, Point at which 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 water 

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

 the tube-foot ; 8, pseudohsemal vessel to the same ; 9, nerve branch to the same ; 10, branch of the 

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

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

 upper intervertebral muscle, enters the vertebral ossicle. 



(c/. 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- 



