42 



MANUAL OF THE MOLLUSCA. 



FORMATION AND GROWTH OF THE SHELL. 



The shell, as before stated, is formed by the mantle of the 

 shell-fish, indeed, each layer of it was once a portion of the man- 

 tle, either in the form of a simple membrane, or as a layer of 

 cells ; and each layer was successively calcified (or hai'dened with 

 carbonate of lime) and thrown off by the mantle to unite with 

 those previously formed. Being extra- vascular it has no inherent 

 power of repair. {Carpenter.) 



The epidermis and cellulai- structm-es are formed by the mar- 

 oin (or collar) of the mantle; the membranous and nacreous 

 layers, by the thin and transparent portion which contains the 

 viscera; hence we find the pearly textm-e only as a lining 

 inside the shell, as in the nautilus, and all the aviculidce and 

 turhinida. 



If the margin of a shell is fractured dming the life-time of the 

 animal, the injury will be completely repaii'ed by the reproduction 

 both of the epidermis and of the outer layer of shell with its pro- 

 per colour. But if the apex is destroyed, or a hole made at a 

 distance from the aperture, it will merely be closed with the 

 material secreted by the visceral mantle. Such inroads are often 

 made by boring worms and shells, and even by a sponge {cliona) 

 which completely mines the most solid shells. In Mr. Gray's 

 cabinet is the section of a cone, in whose apex a colony of litJiodomi 



Fig. 25. 



SecUon of a cone jjeifoiated Oy hthodoml. 



