196 NATURAL THEOLOGY. 



earth is dissolved in the acid, a gelatinous sub- 

 stance of the form of the shell remains. 



Crustaceous animals, such as the lobster and 

 crab,* have their shell formed of the same sub- 

 stances as the testacea, but vsrith the addition of 

 phosphate of lime to the carbonate of lime. A 

 question may arise, How do these animals grow ? 

 It is said that they cast their shells and remain 

 retired until a new shell is secreted ; and Reau- 

 mur has given a very particular account of the 

 process of separation in the cray-fish. Naturalists 

 have not found these cast-off shells. If they be 

 not cast, the animals must, at a particular season, 

 have their shells so softened as to permit sudden 

 expansion of their bodies within ; yet it would be 

 difficult to say by what internal means this shell 

 could be thus softened and made pliant. We 

 presume the reason that the shells of the Crustacea 

 are not found in our museums is because they are 

 not thrown off at once, but that the portions are 

 detached in succession. In these Crustacea we 

 find an approximation to bone, inasmuch as the 

 shell is articulated, and has certain processes di- 

 rected inwards to which the muscles are attached. 



The hardening material of bone is the phos- 

 phate of lime ; and this earthy substance is not 

 merely united with cartilage or gelatinous matter, 

 but membranes and vessels enter into tlie compo- 

 sition of bone. Bone is not excreted, or thrown. 



* Vol. i. p. 297. 



