STRUCTURE OF SEA-URCHIN. 233 



the number and form of the divisions of which 

 the shell consists, are the same both in the full- 

 grown urchin and in the young animal ; they 

 differ only in size. By this beautiful provision 

 the sphere is gradually enlarged, without any 

 alteration of its form or of the relative position 

 and size of its various parts. How admirable 

 this arrangement ! How completely adapted to 

 the end in view ! Were we to suppose there ex- 

 isted a necessity for a certain bridge gradually to 

 increase in size up to a particular point, we 

 could imagine no other available plan than this. 

 Either the whole structure must be taken to 

 pieces and built with stones either larger or 

 more numerous, or the stones originally employed 

 must increase in breadth and length by the addi- 

 tion of new matter at their sides and ends a 

 process, however, far beyond the limits of human 

 contrivance. 



Other parts of this creature's structure are no 

 less striking and interesting than those now de- 

 scribed. If the shell be denuded of the spines, it 

 will appear to be from top to bottom marked out 

 by five double rows of small holes into ten spaces 

 shaped somewhat like the gores into which paper 

 is cut in forming a balloon. Each of these spaces 

 is studded with rows of minute hemispheres. 

 These little points, which seem when the shell is 

 divested of its spines to be merely ornamental, 

 are a portion of a piece of mechanism truly ad- 

 mirable. It is to one of these that each of the 

 spines is fixed when the animal is alive. Each 



