VIII 



EOHINODERMATA RADIAL ORGANS 



411 



Eehinoidea (Fig. 353). In a transverse section of an ambulacral 

 area we find : 



(a) The outer body epithelium (11). 



(b) The cutis, almost entirely calcified, as ambulacral plates (8). 



(c) The epineural sinus (12). 



(d) The radial nerve trunk (17). 



(e) The subneural sinus pseudohsemal canal (14). 



FIG. 354. Transverse Section through the arm of an Asteroid, diagrammatic. 1, Ridges of 

 the deeper oral nervous system ; 2, radial canal of the water vascular system ; 3, continuation of the 

 axial organ in the arm ; 4, radial nerve ridge of the superficial oral system ; 5, pseudohsemal canal ; 

 6 and 7, branches of the pseudohsemal system running to the tube-feet ; 8, pedicellaria ; 9, spine ; 

 10, genital aperture ; 11, branchial vesicle (papulla) ; 12, sessile pedicellaria ; 13, continuation of the 

 body cavity into the branchial vesicle ; 14, brachial diverticulum of the stomach ; 15, circular sinus 

 of the schizocoel round the branchial vesicle ; 16, supramarginal plate ; 17, inframarginal plate ; 18, 

 adambulacral plate ; 19, marginal canal of the pseudohsemal system ; 20, canal connecting it with 

 the body cavity ; 21, endothelium of the body cavity ; 22, genital sinus of the coelom ; 23, gonad 

 (ovarium) ; 24, mesenteries of the diverticula of the stomach ; 25, ampulla canal of the water vas- 

 cular system ; 26, ampulla ; 27, tube-feet canal ; 28, upper and lower transverse muscles of the 

 ambulacral skeleton ; 29, motor branches of the deeper oral nervous system ; 30, ambulacral plates ; 

 31, brachial cavity (coelom) ; 32, apical longitudinal muscle of the arm ; 33, nerve ridge of the apical 

 nervous system. 



(/) The radial blood vessel (15). 



(g) The radial canal of the water vascular system (16). 



(h) The endothelium of the body cavity (13). 



The figure at the same time illustrates the relation of the tube-feet 

 to their ampullae, the double pores, etc. 



Asteroidea (Fig. 354). In a transverse section through the lower 

 (oral) wall of an Asteroid arm we find, from without inwards : 



