SYSTEMATIC DISTRIBUTION. 75 



cous secretion, and again lines it as before. The mu- 

 cous secretion first deposited forms the outer coat 

 of the shell, and is of use in protecting it from injury, 

 while the mucous matter mixed with lime, which is 

 placed within it, forms the substance of the shell itself. 

 This deposition of mucus, and of mucus mixed with 

 calcareous matter, goes on as the animal grows and 

 feels the want of a larger shell for its protection : the 

 shell is, in fact, moulded on the body of the ani- 

 mal itself, as the body grows; and for this reason 

 any irregularity in the body is moulded in the 

 shell. 



The animal has the faculty, also, of mending any 

 break or injury that its shell may have received, if it 

 is not of such a magnitude as to derange all the func- 

 tions of the animal itself; and it mends them in the 

 same manner as it forms its shell ; that is to say, 

 by depositing first a coat of animal matter, and then 

 lining it with mucous matter, mixed with chalk, to 

 harden it. But as the animal is usually very desirous 

 of getting the repairs done as quickly as possible, and 

 is most probably damaged by the injury it has re- 

 ceived, these repairs are generally much more roughly 

 executed than the shell itself, and commonly destitute 

 of regular colour. 



The particles which vary the colour of the surface 

 of the shell are deposited while the shell is being 

 increased in size, immediately under the outer mu- 

 cous coat ; and as these particles are also secreted by 

 peculiar glands, the colour is always situated in a 

 particular manner on each species, the glands being 

 gradually enlarged, and gradually separated, but 

 not changed in position by the growth of the shell. 

 E 2 



