428 



COMPARATIVE AN'ATOMY 



CHAP. 



calcareous ring, in each tentacle canal a muscular membrane forms a semilunar 

 valve -which projects from the Avail with its concave side directed forwards (orally). 



Fig. 365.— Section througli tlie oral region of an Actinopod, in the direction of the principal 

 (longitudinal) axis. On the riglit, the plane of the section is radial ; on the left, almost interradial. 

 1, Cutis ; 2, body epithelium ; 3, oral tentacle, cut ott' ; 4, water canal of the oral tentacle ; 5, blood 

 vessel of the oral tentacle ; 6, tentacle nerve ; 7, circular nerve ; S, oral portion of the coelomatic 

 periesophageal sinus ; 9, mouth ; 10, oesophagus ; 11, pericesophageal sinus ; 12, interradial 

 portion of the calcareous ring ; 13, "water vascular ring ; 14, blood vascular ring ; 15, ventral 

 intestinal vessel ; 16, intestinal epithelium; IT, Polian vesicle ; IS, ampulla of the oral tent-acle ; 

 lit, endothelium of the body cavity ; 20, circular musculature of the body wall ; 21, body cavity ; 

 22 and 26, radial blood vessels ; 23, radial nerve trunk of the superficial system ; 24, radial epineural 

 canal ; 25, radial periha^mal canal ; 27, radial canal of the water vascular system ; 2S, longitudinal 

 muscles ; 20, commencement of tlie radial canal of the water vascular system ; 30, radial portion of 

 the calcareous ring ; 31, I'etractor muscle ; 32, dorsal intestinal vessel. 



This valve prevents the water vascular fluid floAvJng back out of the tentacles into 

 the circular canal. 



The wall of the tentacle canals consists, from without inwards, of : (1) the endo- 

 thelium of the body cavity'; (2) a longitudinal muscle layer ; (3) a layer of connective 

 tissue ; (4) a circular muscle layer ; (5) an inner epithelium. 



