ON DESIGN IN ANIMAL BODIES. 195 



shell, rude as it seems, is not a thing of chance. 

 Since the shell is a cast of the body of the animal, 

 the peculiar shape must have been given to the 

 soft parts, in anticipation of that of the shell — an 

 instance of prospective contrivance. 



That the general conformation of the shell 

 should have relation to what we may term its 

 function, will be less surprising when we find a 

 minute mechanical intention in each layer of that 

 shell. We should be inclined to say that the 

 earthy matter of the shell crystallizes, were it not 

 that the striated or fibrous appearance differs in 

 the direction of the fibres in each successive stra- 

 tum — -each layer having the striae composing it 

 parallel to one another, but directed obliquely to 

 those of the layer previously formed, and the 

 whole exhibiting a strong texture arranged upon 

 well-known mechanical principles. 



Shell is not alive, as true bone is. If the shell 

 of any of the testacea be broken, the surface of 

 the animal secretes a new shell ; not, however, by 

 the concretion of mucus, but by the regular secre- 

 tion of a substance combined of earthy and gela- 

 tinous matter.* Delicate experiments have been 

 made by steeping shells in diluted nitric acid, by 

 which it is shown that the carbonate of lime is 

 the earthy material of shells ; and that, when that 



* We owe our knowledge of the formation of shell to the great 

 French naturalist Reaumur, who, by ingenious experiments, 

 showed the distinction of shell and bone, and that the former was 

 secreted from the surface of the animal. 



