198 



ECHINODEEMATA. 



Fig. 98. 



masticating apparatus : ten others (h 7i), arising in pairs from the 

 middle of the interspaces between the arches, are connected with the 

 bifurcated extremities of the slender curved processes (e e), each of 

 these receiving a muscle from two contiguous spaces ; and, from the 

 length of the levers upon which these muscles act, we may well con- 

 ceive the force wherewith they will influence the motions of the whole 

 mass of the jaws. 



(517.) Such is the complex structure of the mouth of Echinus escu- 

 lentus a piece of mechanism not less remarkable on account of the 

 singularity of its construction, than as exhibiting an example of the 

 sudden development of a dental system, whereof not a vestige is visible 

 in any of the preceding Echinoderm families. In others of the Echi- 

 nida3 having the shell much depressed, the dental lantern is modified in 

 form and proportionately flattened, but the different parts are essen- 

 tially similar to those we have described. 



(518.) The oesophagus (fig. 98, d) is continued from the termina- 

 tion of the central canal that traverses 

 the axis of the lantern, and, after a 

 short course, terminates in a much 

 wider portion of the digestive tube, 

 into which it opens on the lateral 

 part of its ca?cal origin, in a manner 

 precisely resembling the communica- 

 tion between the large and small in- 

 testines of man. 



(519.) The dilated alimentary tube 

 (c) presents no separation into 

 stomach and intestine, but is con- 

 tinued in a winding course around 

 the interior of the shell, which it twice 

 encircles, and, becoming slightly con- 

 stricted, terminates at the anal orifice 

 (i). The walls of the intestine are 

 extremely delicate, although they 

 may be distinctly seen to contain 

 muscular fibres and are covered with 

 innumerable vascular ramifications. 

 The external tunic of the whole canal 

 is derived from the peritoneum, that 

 lines the entire shell, invests the 

 dental lantern, and forms sundry 

 mesenteric folds as it is reflected 

 upon the other viscera. 



(520.) The system of vessels provided for the circulation of the blood 

 has been differently described by different authors a circumstance by 



Alimentary canal of Echinus esculenluv : 

 a, interior of shell ; b, ambulacral fora- 

 mina ; c c, intestinal canal ; d, commence- 

 ment of oasophagus from the base of the 

 " lantern of Aristotle"; e, heart; f, g, vas- 

 cular trunks following the course of the in- 

 testine. 



