34 



MAmiAL OF THE MOLLTTSCA. 



FORMATION AND GROWTH OF THE SHELL. 



The shell, as before stated, is formed by the mantle ; indeed, 

 each layer of it was once a portion of the mantle, either in the 

 form of a simple membrane or as a layer of cells ; and each 

 layer was successively calcified (or hardened with carbonate of 

 lime) and thrown off by the mantle to unite with those pre- 

 viously formed. Being extravascular it has no inherent power 

 of repair. — {Carperder.) 



■ The epidermis and cellular structures are formed by the 

 margin (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 texture only as a lining 

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

 turbinidcB. 



If the margin of a shell is fractured during the lifetime of 

 the animal, the injury will be completely repaired by the re- 

 production both of the epidermis and of the outer layer of shell 

 with its proper 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 shell, and even by a sponge 

 {cliona), which completely mines the most solid shells. In Dr. 

 Gray's cabinet is the section of a cone, in whose apex a colony 



Fig. 25. Section of a Cone perforated by Lithodomi, 



of lithodomi had settled, compelling the animal to contract 

 itself faster than it could form shell to fill up the void. 



Lines of growth. So long as the animal continues growing 

 each new layer of shell extends beyond the one formed before 

 it ; and, in consequence, the external surface becomes marked 

 with lines of growth. During winter, or the season of rest 



