STRUCTURE OP AN URCHIN SHELL. 179 



without close inspection ; but the shell may easily be, 

 if allowed to macerate for some days iu fresh water, 

 broken up into its component parts. These will be found 

 to be of various sizes and shapes, in different parts of the 

 shell, but nearly all are pentagonal, and one so nicely 

 fitted to the other, that no minute space is left without 

 its covering. The very complex structure of this shell 

 may at first seem to be a waste of skill, an expenditure 

 of contrivance uncalled-for by the wants of the creature. 

 But we may be assured that there is no such waste in 

 Nature j and, in the present instance, the structure is 

 easily accounted for, and may be shown to be the best 

 which could be devised to answer the required purpose. 

 It is required to form a globose shell sufficiently hard 

 to afford protection to the soft parts of the animal, and 

 so constructed that it will gradually enlarge, with the 

 growth of the creature, without any alteration of form. 

 A simple crust would not answer these purposes, for, 

 once formed and hardened, it would be incapable of 

 further growth. A crust composed of a multitude of 

 pieces, as this is, completely answers the purpose ; for 

 the whole body may be caused to increase in growth, 

 with the greatest regularity, by constant minute addi- 

 tions to the edges of the several pieces. And this is 

 the method by which the shell of the Urchin does 

 increase. If we examine a living Urchin, we shall find 

 that every portion of the surface of the shell, and even 

 of the spine, is coated over with a delicate living mem- 

 brane, and that this membrane insinuates itself between 

 each of the pieces of the shell, however closely pressed 

 together they appear. In this membrane resides the 



