414 THE MICROSCOPE. 



being constantly made to the free edge by the secretion of 

 calcareous matter at the margin of the mantle. The 

 delicate membranous part of the mantle, which lines the 

 interior of that part of the shell inhabited by the animal, 

 has, also, the power of secreting a thin layer of shelly 

 matter upon its inner surface. This is frequently of a 

 pearly lustre ; and in many bivalves a new layer of this 

 substance is deposited at the time when the size of the 

 shell is increased by additions to its margins, for, it 

 must be observed, that the formation of new shell is not 

 constantly going on, but appears to be subject to periodical 

 interruptions, as indicated by lines on the surface of the 

 shell ; which are called lines of growth. In many cases, the 

 margin of the mantle, instead of being even, presents lobes 

 or tubercles, these produce corresponding irregularities, 

 ribs, tubercles, or spines, on the surface of the shell. 



Mr. Bowerbank says, " Shell is developed from cells 

 that in process of growth have become hardened by the 

 deposition of calcareous matter in the interior." This 

 earthy matter consists principally of carbonate of lime, 

 deposited in a crystalline state; and in certain shell, as in 

 that of the common Oyster (fig. 205, No. 2), from the 

 animal-cell not having sufficiently controlled the mode of 

 deposition of the earth particles, they have assumed the 

 form of perfect rhomboidal crystals. 



The shell of the genus Pinna, " Wing-shells," is com- 

 posed of a series of hexagonal cells filled with transparent 

 calcareous matter, seen at fig. 198, No. 2, the outer layer 

 of which can be split up into prisms, like so many basaltic 

 columns; as at No. 1. 



Organs of sense are possessed by this class in an ad- 

 vanced condition of development. In the Scallop (Pecten), 

 for example, eyes occur in great numbers, placed among 

 the tentacles on the borders of the mantle. In other 

 genera, the eyes are differently placed, in Pinna on the fore 

 part of the mantle, and around the siphon-orifices in Pholas 

 and Solen. In the Cockle (Cardium) the short siphons are 

 surrounded with an extraordinary number of tentacles, 

 capable of protrusion, each of which bears a pretty little 

 eye; these are beautiful objects under the microscope. 

 Cockles are able to perform vigorous leaps by means of a 



