SHELL. 



[ 576 ] 



SHELL. 



pressure. Transverse sections of the pris- 

 matic structure exhibit a cellular appear- 

 ance (PL 37. figs. 4 & 11 a), and a somewhat 

 similar appearance is presented by perpendi- 

 cular sections (PL 37. figs. 5 & 11 b). The 

 prisms are pretty easily separable in the lines 

 of mutual contact, and often form several 

 superimposed strata. They frequently con- 

 tain pigment, either uniformly diffused 

 through their substance, or in granules. 

 They also sometimes appear transversely 

 striated. 



The inner laminated portion, which some- 

 times constitutes the entire shell, is either 

 white or presents the brilliant iridescent 

 tints of nacre or mother-of-pearl. It is often 

 called the nacreous portion, or nacre, and 

 when polished forms the mother-of-pearl of 

 the shops. Under the microscope it exhibits 

 a number of fine lines or grooves, running 

 in various directions, and probably corre- 

 sponding to the edges or intersections of the 

 strata or lamina, of which this portion of 

 the shell is composed; and it is to the inter- 

 ference of light ensuing at the surfaces of 

 these grooves that the iridescent colours are 

 usually owing. 



In some shells there are tubes traversing 

 the substance perpendicularly (PL 37. fig. 7) 

 or obliquely, or forming branched horizontal 

 channels (PL 37. fig. 9 a, b) ; in the latter 

 case they are sometimes connected with 

 rounded cavities (PL 37. fig. 9 a). 



In some Gasteropoda, as Cyprcea, the 

 outer portion of the shell consists of three 

 layers of similar prismatic structure, but with 

 the prisms in each layer in alternately con- 

 trary directions. The same may be seen in 

 some of the outer layers of oyster-shell, 

 except that the prisms are nearly horizontal 

 or slightly oblique. But in the Acephala 

 generally, the structure corresponds to the 

 inner portion of that of the Cephalophora. 



Shell consists of an organic basis, in which 

 calcareous matter, principally composed of 

 carbonate of lime, is deposited ; and by 

 digesting it with dilute muriatic acid, the 

 latter may be removed, an organic cast of 

 the original being left. On treating a thin 

 plate of nacre in this way, Dr. Carpenter 

 found that the iridescent colours remained 

 visible until the membrane was stretched, 

 and the supposed folds obliterated when 

 they vanished ; hence this author concludes 

 that the edges of the folds were the cause of 

 the interference of light producing the 

 colours. It appears to us, however, objec- 

 tionable to this view, that the same structure 



and colour is produced by laminated calca- 

 reous and organic matter artificially formed; 

 that they are also present after the edges of 

 the folds must have been ground away, as 

 in sections ; and that the colours, in the 

 instance mentioned, might have been those 

 of a thin plate, and some of the colours of 

 iridescent shell are known to be those of 

 thin plates. It may be stated here that 

 Dr. Carpenter considers the lines or striae in 

 nacre to be produced by the edges of folds 

 of a single layer of membrane, arranged so 

 as to lie over each other in an imbricated 

 manner. The same author views the shell 

 of the Mollusca as corresponding to the epi- 

 dermis of the higher animals calcified. 



The outer prismatic layers of shell are 

 secreted by the borders or margins of the 

 mantle, whilst the inner laminated portions 

 arise from the outer surface. The growth 

 of shell is not uninterrupted or constant, but 

 periodical; hence the laminated arrangement 

 of its constituents. 



In some portions of the shell of the oyster, 

 &c., the calcareous matter assumes the form 

 of distinct rhomboidal or hexagonal crystals 

 (PL 37. fig. 10). These appear to be depo- 

 sited in the inner laminated portion ; and 

 when detached, they leave angular spaces 

 corresponding to them in form. In the 

 tooth of the shell of My a, groups of radiating 

 prisms are present, forming an elegant 

 microscopic object. 



The prisms existing in the outer portion 

 of shells have been supposed to represent 

 cells filled with calcareous matter ; they 

 have also been regarded as consisting of 

 aggregations of epidermic cells, the transverse 

 striae (in Pinna) corresponding to thickenings 

 of the cell-membranes where the layers come 

 into contact, and the folded membrane has 

 been compared to a basement membrane. 

 It is probable, however, that shell should be 

 regarded as a simple secretion from the 

 mantle, and as corresponding in structure to 

 egg-shell. 



Shell of the Crustacea. The hard portion 

 of the integument of the Crustacea, alluded 

 to at p. 175, possesses a laminated structure, 

 corresponding to periods of growth, and 

 giving rise to the appearance of transverse 

 parallel lines in a perpendicular section 

 (PL 37. fig. 1 6). The substance is traversed 

 by numerous straight or slightly wavy, very 

 slender tubes (PL 37. fig. 16), resembling 

 those of dentine. 



Shell of Echinodermata. The perforated 

 structure of the homogeneous basis forming 



