410 COMPARATIVE ANATOMY CHAP. 



radial region of the body wall of an actinopodan Holothmian we find, 

 proceeding from without inwards : 



(a) The outer body epithelium (10). 



(b) The cutis, or the connective tissue layer of the body wall, wit 



the calcareous corpuscles (9). 



(c) The epineural canal (16). 



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



(e) The radial nerve trunk of the deeper oral system (7). 

 (/) The subneural pseudohsemal canal (18). 



(a) The radial blood lacuna (radial blood vessel) (6). 



FIG. 353. Transverse Section through a radial region of the body wall of an Echinoid, partly 

 diagrammatic. 1, Ampulla, traversed by muscle filaments ; 2 and 3, the two canals traversing the 

 test and connecting the ampulla and the tube-foot canal (5) ; 4, circular nerve in the terminal disc 

 of the tube-foot ; 5, tube-foot canal ; 6, nerve of the tube-foot ; 7, integumental nerve ; 8, calcareous 

 substance of the ambulacral plate ; 9, nerve plexus in the depths of the body epithelium ; 10, suture 

 between two plates of the two contiguous rows of ambulacral plates ; 11, body epithelium ; 12, 

 epineural canal ; 13, endothelium of the body cavity ; 14, pseudohsemal canal ; 15, radial blood 

 vessel ; 16, radial canal of the water vascular system ; 17, radial nerve strand ; 18, lateral canal of 

 the radial canal of the water vascular system to the ampulla. 



(h) The radial canal of the water vascular system (5), and the tube- 

 foot canal branching from it transversely (11), and finally 

 also the ampulla of the tube-foot (8). 



(i) The circular musculature of the body (2). 



(k) The longitudinal musculature of the body (3). 



(1) The endothelium of the body cavity (1). 



The figure also illustrates the relation of a tube-foot to its canal 

 and ampulla. 



This description does not apply to the Paradinopoda (Synaptitto) in 

 so far as, in these latter, the radial canals of the water vascular system 

 are altogether wanting. 



