HOW THE SHELL GROWS. 219 



a semicrystalline arrangement, progressively increase 

 its dimensions. But the continual augmentation in 

 size which is thus going on simultaneously in all 

 parts, is attended with no change in the mathematical 

 figure of any given piece of the shell, so that the 

 whole fabric gradually expands without in any degree 

 altering its form or relative proportions, until it has 

 acquired the mature dimensions belonging to its 

 species. 



If we remove the spines of a young specimen re- 

 cently dead, we shall then observe, that amongst the 

 plates situated near the opposite orifices of the shell 

 and the plates that bear spines, there are some less 

 fully developed, irregular in form, and taking their 

 place amongst the perfectly formed plates only in 

 proportion as they gradually attain to a larger size. 

 In that region of the body where this increase takes 

 place, the membrane which unites all the plates, and 

 spreads itself over their surface, is softer and more 

 spongy than it is in those parts where the plates are 

 consolidated and immoveable. It is, in fact, this spongy 

 mass that deposits the calcareous matter of which the 

 plates are composed, and the spines shoot out in the 

 centre, almost in the same manner as the horns of a 

 stag. They do not become moveable until they have 

 attained a certain degree of development, and there 

 is a period in their growth after which their size does 

 not increase. Those, moreover, which drop off acci- 

 dentally are replaced by others, formed as the pre- 

 ceding ones had been, by the membrane which covers 

 the plates. We may always indeed observe in a single 

 specimen all the gradations of increase, from that of 



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