530 THE MICROSCOPE. 



closed within the mantle, are called Nudibranchiata ; an 

 example of this family is seen in fig. 2-48, Aplysia ; but it is 

 remarkable that most of them are provided with a small 

 shell when they first quit the egg. In the shell-bearing 

 or Testaceous Mollusca, this embryonic shell, which often 

 differs greatly in shape and texture from the shell of the 

 mature animal, is, however, a commencement of the latter ; 

 additions being constantly made to the free edge by the 

 secretion of calcareous matter at the margin of the mantle. 

 The delicate membranaceous part of the mantle, which 

 lines the internal portion 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 con- 

 stantly 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 of tubercles ; these produce corresponding irregula- 

 rities, — ribs, tubercles, or spines, — on the surface of the 

 shell. 



Dr. 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. 247, 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 in fig. 240, No. 2, the cuter layer 

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

 columns ; as at No. 1. 



Organs of sense are possessed by some of this class in an 

 advanced state of development. In the Scallop (Pecten), 

 tor example, eyes occur in great numbers, placed among 



