MOLLUSCA. 255 



the earlier observers. The Pholades, however, which in other respects 

 are true bivalve MoUusca, are furnished with a pair of accessory plates 

 in the neighbourhood of the hinge; whilst the Chitons, fig. 106, a small 

 but singular group of MoUusca nearly allied to the univalve limpets, 

 have an oval shell composed of eight movable plates, which gives, them a 

 great resemblance to enormous woodlice ; and they have been regarded 

 as forming a sort of transition towards the articulated division. 



Many MoUusca are not furnished with a shell, or have only a small 

 calcareous plate enclosed within the mantle. These are called naked 

 MoUusca, an example of this family is seen in the frontispiece, Aplysia; 

 but it is remarkable, that most of them are provided with a small shell 

 at their first quitting the egg. In the shell-bearing, or testaceous Mol- 

 lusca, this embryonic shell, which often differs greatly in shape and 

 texture from the shell of the mature animal, forms the commencement 

 of the latter, additions being constantly made to its free edge by the 

 secretion of calcareous matter at the edge 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, however, the power of secret- 

 ing a thin layer of shelly matter upon the inner surface. This is 

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

 substance is deposited at the same time that 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, indicated by lines on the surface 

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

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

 which 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 (Plate VI. No. 8), from the animal-cell not 

 having sufficiently , controlled the mode of deposition of the earth par- 

 ticles, they have assumed the form of perfect rhomboidal crystals. 



The Oyster belongs to the class called Acephahcs, or non-headed, 

 a» it has no distinct head. The gills, or breathing apparatus, form 

 what is commonly called the beard of the oyster. The creature is 

 attached by strong muscles to its shell. The mouth of the oyster is 

 a mere opening in the body, without jaws or teeth ; and its food con- 

 sists of nourishing substances contained in the water, and which are 



