113 



Echinidce, — This order is filled by those animals which 

 have been hitherto named echini. 



The echinus is one of the radiated echinodermata of 

 Lamarck, and is distinguished from the preceding section of 

 this order, the stelleridce, by the mouth and vent being 

 separate : the body, which is short, variously formed, and 

 generally quinquepartite, is inclosed in a crustaceous, 

 porous, and spiniferous covering, formed by numerous po- 

 lygonal assulae, or little plates joined by finely serrated 

 vertical sutures, crossed by others rather more linear : the 

 whole is still farther connected by an investing cuticle. 



The crust or shell readily separates into five triangular 

 sections, one of which is represented, PI. x. fig. 6 : each of 

 these is divisible into four sections ; two larger, a, a. termed 

 areas, ornamented with roundish tubercles on which are 

 articulated the larger spines ; and two smaller, b. b. termed 

 ambulacra^ which are perforated by numerous pores, from 

 which proceed retractile tubules, terminating in strongly 

 adhering sucker-formed feet. > Each of these five triangular 

 sections dividing thus into four smaller, the whole crust, it 

 appears, is formed of twenty vertical sections. 



Besides the almost innumerable pores from which the 

 feet proceed, there are other larger pores at the upper part 

 of each section, for the exclusion of the ovae: two large 

 openings also exist for the mouth and vent. The mouth is 

 always on the lower part, and in most kinds has in its 

 middle five teeth, collected together and terminating in a 

 firm point, bearing the appearance of enamel : these teeth 

 are fixed in a ' testaceous craticular apparatus, which has 

 been termed Diogenes' lantern. The vent is variously situ- 

 ated, and is surrounded by tubercles, from which generally 

 proceed numerous spines. 



The fossil remains of echini are frequently found in so 

 perfect a state of preservation, as must suggest many sub- 

 jects deserving earnest inquirj- ; the most important of 



a. 



