VIII 



EGHINODERMA TA—RA DIA L ORGANS 



411 



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

 area we find : — 



{a) The outer body epithelium (11). 



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



{c) The epineural sinus (12). 



{d) The radial nerve trunk (17). 



(e) The subneural sinus — pseudohaemal canal (14). 



Fio. 354. — Transverse Section through the arm of an Asteroid, (liagraiiimatic. 1, Ridges of 

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

 axial organ in the ar)n ; 4, ladial nerve ritlge of the superficial oral system ; 5, pseudohsemal canal ; 

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

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

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

 of the schizoccel round the branchial vesicle ; 16, supramarginal plate ; 17, infraniarginal plate ; IS, 

 adainbulacral plate ; 19, marginal canal of the pseudohwmal 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). 



(A) 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. 



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

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



