sEA-nicHiNs. 167 



few, on seeing them, would have any idea of their appear- 

 ance in a living state, clothed with spines. The materials 

 however of which they are composed, consisting of hun- 

 dreds of plates nicely fitted together by their edges, and 

 beautifully arranged in symmetrical order, with their spine- 

 bearing tubercles, form in them.selves unitedly an object of 

 admiration. And the manner in which the whole body 

 increases in bulk is also interesting in the highest degree. 

 The plates are united by a membrane covering the whole, 

 and inserted between the edges. This membrane secretes the 

 calcareous matter of which they are composed. The matter 

 is deposited on the edges. Every large or small angle of 

 every large or small plate, must receive its proper propor- 

 tion of the deposition in the same time ; else, the form of 

 the whole W'ould not be retained. As in a line of soldiers 

 abreast turning an angle, the outer man must march the 

 fourth of a large circle, and the centre man that of a 

 smaller one, while the inner man only turns his own body, 

 so the upper and lower plates of the Echinus will require 

 very little addition to their edges to be kept in a radiating 

 line with those of the centre, which require large additions 

 in the same time, to make their share of the middle circum- 

 ference, and all these proportions must always be kept, or 

 the form of the wiiole is lost. 



