20 THE SHELL. 



caused by the wants of the mollusks themselves, which in waters 

 not sufficiently charged with lime are compelled to devour the 

 unoccupied and unprotected parts of their neighbors' houses to 

 obtain material for the enlargement of their own. * 



Formation and Growth of the Shell. The shell, as before stated, 

 is formed b}^ 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 imite with those previously formed. Being extravas- 

 cular it has no inherent power of repair. 



The epidermis and cellular strvictures 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 Aviculidse and 

 Turbinidse. 



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

 animal, the injury will be completely repaired by the reproduction 

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

 proper color. 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. 



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 which 

 corresponds to it, shells cease to grow; and these periodic resting- 

 places are often indicated by interruptions of the otherwise 

 regular lines of growth and color, or by still more obvious signs. 

 It is probable that this pause, or cessation from growth, extends 

 into the breeding season ; otherwise there would be two periods 

 of growth and two of rest in each year. In many shells the 

 growth is uniform ; but in others each stage is finished by the 

 development of a fringe, or ridge, varix (xlvi, 54, 55), or of a 

 row of spines, as in Tridacna and Murex. 



Adult Characters. The attainment of the full growth proper 

 to each species is usually marked by changes in the shell. 



Some bivalves, like the oyster and Gryphsea, continue to 

 increase in thickness long after they have ceased to grow out- 

 wards ; the greatest addition is made to the lower valve, especially 

 near the umbo ; and in the Spondylus some parts of the mantle 



* Fischer, Actes Linn. Soc. Bord., xviii. 



