168 



ECHINODEEMATA. 



ternal covering is very different ; for these creatures seem completely 

 encased in a dense calcareous shell, composed of numerous angular pieces 

 accurately fitted together and incapable of movement. The Scutellce, 



Fig. 84. 





x \\ I 



Scutella. 



moreover, bury themselves beneath the surface of the sand a situation 

 in which suckers would be of little use, but for which these animals 

 are admirably adapted by a contrivance not less calculated to excite the 

 admiration of the observer. The exterior of the shell is entirely covered 

 with minute appendages resembling, when seen with the naked eye, 

 delicate hairs ; but these, when examined under a microscope, are found 

 to be spines of most elaborate structure, as is evident from the mag- 

 nified view of one represented in the annexed figure (fig. 84). Innu- 

 merable as these spines are, every one of them is articulated to the shell 

 by a kind of ball-and-socket joint, and susceptible of being moved in 

 all directions ; so that by their combined eiforts the Scutella can speedily 

 bury itself, either for the purpose of procuring food, or of eluding 

 observation. 



(441 .) From the flat Scutellce, the passage to the globose ECHTNIDJE 

 is uninterrupted ; and a beautiful series of connecting forms (many still 

 existing as living species, but a still greater number found only in a 

 fossil state) demonstrate the progressive expansion of the shell, and 

 its conversion into the spherical figure seen in the Echinus esculentus 

 (fig. 85). The Echinus in shape resembles an orange, its dense calca- 

 reous crust enclosing the viscera within its cavity, while the locomotive 

 apparatus is placed upon the external surface. The mouth is a simple 

 orifice in the shell, placed at one extremity of its axis ; and through it, 

 as represented in the figure, the points of five singular teeth project 



