290 GROWTH OF THE ECHINUS. 



more lively and walkative, (if I may be allowed the 

 expression) than their more. corpulent brethren. 



The inflexible, mail-like crust, or shell, as it is 

 commonly called, of the Echinus is perhaps one of 

 the most marvellous objects on which the eye can 

 rest. Although at first sight it appears to be a solid 

 calcareous box, it is in reality composed of several 

 hundred pentagonal plates, 1 of various sizes, so closely 

 dove- tailed together that their marks of junction are 

 scarcely perceptible. Upon a superficial examina- 

 tion we are apt (most erroneously) to consider this 

 wonderful piece of work to be more elaborate than 

 the wants of the animal demand. The fact of the 

 Lobster or Crab throwing off its entire shell at certain 

 seasons, to admit of the increased growth of the animal 

 is a truly marvellous phenomenon, still, it would more 

 excite our wonder were we to find that, instead of 

 being cast away at all, the hard, inelastic envelope 

 which surrounds the bodies of crustaceans was made 

 to swell or expand proportionately with the soft 

 parts of the animal ! Now, the mosaic-like shell of 

 the Sea-Urchin, though built up, as before stated, of 

 several hundred pieces, is by a beautiful process 

 slowly and imperceptibly enlarged correspondingly 

 with the growth of the animal. 



The gradual enlargement of the Echinus shell takes 

 place in the following manner: 



1 In a specimen that I examined, and then carefully took to pieces, there were 

 exactly 1780 plates. 



