428 



COMPARATIVE ANATOMY 



CHAP. 



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

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



FIG. 3ti5. Section through the oral region of an Actinopod, in the direction of the principal 

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

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

 vessel of the oral tentacle ; 6, tentacle nerve ; 7, circular nerve ; 8, oral portion of the ccelomatic 

 perioesophageal sinus ; 9, mouth ; 10, oesophagus ; 11, perioesophagcal sinus ; 12, interradial 

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

 intestinal vessel; 16, intestinal epithelium; 17, Polian vesicle; 18, ampulla of the oral tentacle; 

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

 22 and 26, radial blood vessels ; -2.3, radial nerve trunk of the superficial system ; 24, radial epineural 

 canal ; 25, radial perihajinal canal ; 27, radial canal of the water vascular system ; 28, longitudinal 

 muscles ; 29, commencement of the radial canal of the water vascular system ; 30, radial portion of 

 the calcareous ring ; 31, retractor muscle ; 32, dorsal intestinal vessel. 



This valve prevents the water vascular fluid flowing 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. 



