14 MOLLUSCA. 



matter in its cells, and becomes hardened into a layer 

 of shell. The epidermis, like that of other animals, is 

 inorganic, and cast off occasionally by the animal. This 

 calcified epithelium or shell-structure is formed either of 

 prismatic cells, filled with calcareous matter, and arranged 

 perpendicularly in layers, or it is composed of thin 

 membranes, which deposit the shelly matter on their 

 surface. When the membranes are finely plaited, and 

 the folds lie one over another, the nacreous or pearly 

 appearance is produced. Sometimes the membranous 

 shell-structure is traversed by tubes, the direction and 

 distribution of which vary in different genera, but usu- 

 ally they form a network. Occasionally the prismatic 

 shell-structure is very large, and the cells are hollow, 

 as in the Rudistes among Bivalves ; this constitutes the 

 cancellated shell-structure. 



The margin only of the mantle has the power of giving 

 origin to the outer layer of the shell, w^hile its whole 

 surface may generate the inner. As the animal increases 

 in dimensions, each new interior layer of shell projects so 

 far beyond the preceding, that the new border composed 

 of the outer layer is simply joined on to the margin of the 

 former one. The colour is situated always in the outer 

 layers, and is furnished by glands in the edges of the 

 mantle ; and the secretion of the colouring-matter, being 

 uninterrupted, or interrupted at regular intervals, produces 

 all the stripes and spots which serve to ornament shells. 



The shell is moulded on the mantle of the animal, so 

 that some of the most distinguishing generic characters 

 may be detected from its examination. The edges of the 

 mantle are often developed into fringes at certain periods, 

 and form the spines and ribs, and all the irregularities 

 seen on the surfaces of shells. 



