VIII EGHINODERMATA— RADIAL ORGANS 411 



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

 area we find : — 



(a) The outer body epithelium (11). 



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



(c) The epineural sinus (12). 



{d) The radial nerve trunk (17). 



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



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5' 



Fig. 354. — Transverse Section tlirough, tlie arm of an Asteroid, ttiagrammatic. 1, Ridges of 

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

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

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

 10, genital aperture ; 11, branchial vesicle (papuUa) ; 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, infra margin a I plate ; 18, 

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

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

 (ovarium); 24, mesenteries of the divei-ticula 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 ampullse, 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 : — 



