190 



ECIILNODERMATA. 



Fig. 94. 



to the kind of sensation whereby we have already seen the Actiniae and 

 other polyps are able to appreciate the presence of light, although ab- 

 solutely deprived of visual organs. 



(498.) The ECHINI, however they may appear to differ in outward 

 form from the Asteridoe, will be found to present so many points of re- 

 semblance in their general structure, that the detailed account we have 

 given above of the organization of the last-mentioned family will throw 

 considerable light upon the still more elaborately constructed animals 

 that now present themselves to our notice. 



(499.) The Echinidce, as we have already observed, differ from the 

 star-shaped Echinodermata in the nature of the integument that encloses 

 their visceral cavity, as 

 well as in the more or 

 less circular or spheri- 

 cal form of their bodies; 

 so that the locomotive 

 apparatus with which 

 they are furnished is 

 necessarily modified in 

 its character and ar- 

 rangement. 



(500.) The shell of an 

 Echinus (fig. 94, l) is 

 composed of innume- 

 rable pieces accurately 

 j oined together, so as to 

 form a globular box 

 enclosing the internal 

 parts of the animal, but 

 perforated at each ex- 

 tremity of its axis by 

 two large openings, one 

 of which represents the 

 mouth, and the other 

 the anus. 



(501.) The calcareous plates entering into the composition of this 

 extraordinary shell may be divided into two distinct sets, differing 

 materially in shape, as well as in the uses to which they are subservient. 

 The larger pieces are recognizable in the figure by hemispherical tuber- 

 cles of considerable size attached to their external surface, adapted, as 

 we shall afterwards see, to articulate with the moveable locomotive 

 spines. Each of these larger plates has somewhat of a pentagonal 

 form, those that are situated in the neighbourhood of the mouth and 

 anal aperture being considerably the smallest, and every succeeding 

 plate becoming progressively larger as they approximate the central 



1. Shell of Cidaris denuded of its spines. 2. A spine arti- 

 culated with its corresponding tubercle : a, section of tuber- 

 cle ; 6 5, capsular ligament ; c, base of spine. 



