170 



ECHINODERMATA. 



Fig. 72. 



but the different parts are essentially similar to those we have 

 described. 



(208.) The oesophagus (Jig. 72, d,) is continued from the termi- 

 nation of the central canal, which traverses the axis of the lantern, 

 and after a short course termi- 

 nates in a much wider portion 

 of the digestive tube, into which 

 it opens on the lateral part of 

 its csecal origin in a manner 

 precisely resembling the com- 

 munication between the large 

 and small intestines of man. 



The dilated alimentary tube, 

 (c,) which presents no separa- 

 tion into stomach and intestine, 

 is continued in a winding 

 course around the interior of the 

 shell, which it twice encircles, 

 and, becoming slightly con- 

 stricted, terminates at the anal 

 orifice of the shell (z). The 

 walls of the intestine are ex- 

 tremely delicate ; although they 

 may be distinctly seen to con- 

 tain muscular fibres, and are 

 covered with innumerable vas- 

 cular ramifications. The external tunic of the whole canal is de- 

 rived from the peritoneum, which lines the entire shell, invests the 

 dental lantern, and forms sundry mesenteric folds as it is reflected 

 upon the other viscera. 



(209.) The system of vessels provided for the circulation of the 

 blood has been differently described by different authors, a circum- 

 stance by no means surprising when we consider the great difficulty 

 of tracing such delicate and extensively distributed canals. Ac- 

 cording to Delle Chiaje, the course of the nutritious fluid is as 

 follows. A large vein runs along the whole length of the intes- 

 tine, from the anus to the oesophagus, where it terminates in a 

 vascular ring surrounding the mouth ; into which, as in Asterias, 

 the contractile vesicle, which he considers to be a receptacle for 

 the nutrient fluid, and the antagonist to the tubular x feet, like- 

 wise opens. The intestinal vein he regards as the great agent 



