605 



SECOND DIVISION, FIRST CLASS.— MOLLUSCA. 



The general account of the organization of the MoUusca, given by Cuvier (pp. 335, 336,) does not 

 require any important alteration or arldition, save in one particular, — tlie structure of the S/iell, which 

 has recently been made Ihe subject of careful microscopic investigation by Dr. Carpenter (Reports of 

 the British Association, 1844 and 1S47,) and Mr. Bowerbank, (Transactions of the Microscopical 

 Society, Vol. I.) 



In order that the structure of Sljell and its relations to the tegumentary covering of other animals 

 should be properly understood, it will be desirable to give a brief account of what is now known of 

 the latter. The skin of Man, and of Vertebrata in general, is essentially composed of two parts, 

 which are entirely distinct from each other in structure and offices, namely, the coriut/i, dermis, cvfis- 

 vera, OT true-skin; and the epidermis, cuticle, or scarf-skin. The true-skin is made up of fibrous 

 tissue, interwoven with a texture made up of an assemblage of blood-vessels, nerves, and lymphatics, 

 which are very copiously supplied to this part. The cuticle, on the other hand, is entirely destitute of 

 vessels or nerves, and is thence said to be extra-vascular. But it is not on that account a mere 

 inorganic glue exuded from the surface of the skin, as some have supposed. For it is made up of an 

 assemblage of celts, resembling those of which the greater part of the fabric of plants is composed, 

 and of which other parts of the animal body contain a large amount. These cells are most distinctly 

 seen on the internal surface of the epidermis, that is to say, on the outer surface of the true skin; 

 it is here that the growth and renovation of the epidermis are effected ; new layers being continually 

 formed to replace those which are worn oif on the exterior. The cells of the outer surface of the 

 epidermis are dried up by the evaporation of their contents, and are flattened into scales, which adhere 

 to one another, so as to form a continuous membrane. This membrane in most animals is continually 

 undergoing renewal ; for as it is worn off from the exterior, it is replaced by the deeper layers, which 

 then come to the surface and assume the characters of those which preceded them ; whilst the layers 

 last formed on the surface of the true skin are pushed outwards by the production of others still newer. 

 "What has been called the rete mucosiim, which was supposed to be a distinct and peculiar layer, con- 

 taining the colouring matter of the skin, and intervening between the true skin and the epidermis, is 

 now known to be nothing else than the newest layer of the epidermis, through the whole thickness of 

 which the colouring matter is diffused. The nails, hair, horns, scales, and teeth, all belong to the 

 category of epidermic appendages ; being originally generated by the growth of epidermic cells on the 

 surface of the true skin, or within a little follicle or bag formed iiy a pit or depression of that surface. 

 These cells, however, subsequently undergo various transformations, especially in the horny textures, 

 by which their original character becomes obscured ; but they may be well seen, in a nearly un- 

 changed state; in the central portion of most feathers, and of many hairs. 



Now the Shell of Mollusca is an epidertiiic structure, which is. formed on the surface of their 

 mantle, or thick spongy muscular skin, just as the cuticle of higher animals is formed upon the cutis 

 vera. In its original state, it appears to consist of cells, similar to those of the ordinary epidermis : 

 but these cells have the peculiar power of tilling thems'tlves as they grow, wifh carbonate of lime, 

 which they draw in from the fluids of the mantle ; and by coming into contact with each other, and 

 adhering closely, they form the solid calcareous shell. In many shells, when the carbonate of lime 

 has been removed by the action of dilute acid, a tenacious cellular membrane is left; the cells being 

 held together by the interposition of horny matter, which gives considerable firmness to the texture. 

 This is the case for example, in Pinna and its allies. In most other cases, however, the horny matter 

 is exuded as a distinct layer on the surface of the calcareous shell ; forming what has been termed (but 

 incorrectly) the epidermis, the more appropriate term being the periostracum. This layer sometimes 

 presents the appearance of cellular structure, but this is probably rather the impression of the layer o 

 true shell beneath ; it is not at all improbable, however, that it is produced by the agency of cells in 

 the first instance, although no trace of structure is usually discernible in it. 



In a large proportion of the Bivalve Shells of the class Acephala, a distinct layer of cellulai 



