SPINES OF THE SEA-URCHIN. 291 



Over the entire surface of the globular shell, 

 spines, and joints of the living Urchin, there exists a 

 delicate membrane that insinuates itself between 

 the pentagonal plates above mentioned, and con- 

 tinually deposits around the edges a certain portion 

 of calcareous matter (carbonate of lime). The same 

 process being also carried on by the fleshy covering 

 that surrounds the spines, &c., it must be evident 

 that so long as the vital power of the animal exists, 

 each plate and spine, still keeping to its original 

 form, must be daily and hourly augmented in size 

 until the Sea-Egg has attained its full and mature 

 dimensions. 



As to how the spines retain their relative posi- 

 tion in each plate, as the latter gradually becomes 

 enlarged, I cannot positively state ; but may be 

 permitted to mention, that, judging from carefully 

 prepared sections of the plates when submitted to the 

 microscope, each spine appeared to my eye to be by 

 some singular process urged along in a kind of groove 

 to its proper place. 



The hedgehog-like spines that surround the 

 globose body of the Sea-Urchin are all moveable 

 at the will of the animal, each prickle being con- 

 nected by a ball-and-socket joint to a pearly tubercle, 

 which acts as the 'socket' on which the 'ball' of 

 the spine revolves. If the spine be removed, a 

 comparatively smooth surface will be left, on which 

 are various sized tubercles systematically arranged. 



