160 



ECHINODERMATA. 



shallow bays frequented by star-fishes, and observed how frequently 

 a bait is taken and devoured by them, will be disposed to admit 

 this; yet to what are we to attribute this power of perceiving exter- 

 nal objects ? It would seem most probably due to some modifi- 

 cation of the general sensibility of the body, allowing of the per- 

 ception of impressions in some degree allied to the sense of smell 

 in higher animals, and related in character to the kind of sensation 

 by which we have already seen the Actiniae and other polyps 

 able to appreciate the presence of light, although absolutely de- 

 prived of visual organs. 



(SOI.) The ECHINI, however they may appear to differ in out- 

 ward form from the Asteridoe, will be found to present so many points 

 of resemblance 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 con- 

 structed animals which now present themselves to our notice. 



The Echinida, as we have already observed, differ from the 

 star-shaped Echinodermata in the nature of the integument which 

 encloses their visceral cavity, as well as in the more or less circular 

 or spherical form of their bodies ; so that the locomotive apparatus 

 with which they are furnished is necessarily modified in its cha- 

 racter and arrangement. 



The shell of an Echinus (Jig. 68, 1) is composed of innu- 



