VIII 



EGHINODERMATA RADIAL ORGANS 



413 



(h) The endothelium of the body cavity (21). 



The figure also shows the relations of the ampullae to the tube-feet 



and their canals, and the organs of the apical side of the arm 



of an Asteriod. 



Ophiuroidea (Fig. 355). In a section through an arm, proceeding 

 from the lower (oral) to the upper (apical) side, we find : 



(a) The body epithelium. 



(b) The ventral shield (5). 



'/J 



Fio. 356. Transverse Section through the arm of a Crinoid, diagrammatic. 1, Radial nerve 

 trunk of the superficial oral nervous system ; 2, radial pseudohEemal canal ; 3, radial canal of the 

 water vascular system ; 4, the paired deeper longitudinal nerves of the arms ; 5, 7, and 11, the three 

 radial sinuses of the brachial coelom ; 0, genital sinus with genital rachis ; 7 (see 5) ; 8, nerve 

 trunk of the apical nervous system ; 9, end of the nerves at the surface ; 10, branch connecting 4 

 and 8 ; 11 (see 5) ; 12, tentacle nerve ; 13, tentacle canal of the water vascular system ; 14, sensory 

 cone on the tentacle ; 15, food groove of the arm. 



(c) The radial epineural canal (6). 



(d) The radial nerve trunk of the superficial oral nervous system (7). 



(e) The radial nerve trunk of the deeper oral system (9). 

 (/) The (subneural) radial pseudohaemal canal (10). 



(g) The radial canal of the water vascular system (20). 



(h) The calcareous mass of the vertebral ossicle (15), which is 



traversed by the tentacle canals (22), and the intervertebral 



musculature (16 and 13). 



