ECHINODE11MATA. 

 Fig. 61. 



143 



pcct, arc always more or less composed of a coriaceous material, or, 

 at least, of solid pieces so articulated together as to permit of con- 

 siderable flexibility ; but in the Echinidso the nature of the external 

 covering is very different, for these creatures are completely en- 

 cased in a dense calcareous shell, composed of numerous angular 

 pieces accurately fitted together and incapable of movement. The 

 Scutellte, 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 which, when ex- 

 amined under a microscope, are found to be spines of most elaborate 

 structure, as may be seen from the magnified view of one represent- 

 ed in the annexed figure (Jig. 61). Innumerable 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 efforts the Scutella can speedily bury itself, 

 either for the purpose of procuring food, or of eluding observation. 

 (184.) From the flat Scutellcc^ the passage to the globose Echi- 

 nidee is most gradual ; 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 gradual expansion of the 

 shell, and its conversion into the spherical figure seen in the Echinus 

 esculentus (Jig. 62). The Echinus in shape resembles an orange, 



