SHELL. 



557 



whilst they are occasionally found in the deli- 

 cate little PTEROPODA, and in the compara- 

 tively gigantic CEPHALOPODA. In this last 

 class, however, the shells are not unfrequently 

 internal; an approach to this arrangement 

 being seen in certain Gasteropoda and Ptero- 

 poda, in which the shells are covered-in by 

 folds of the mantle, whilst really external to 

 the body. In the articulated series, the pre- 

 sence of a shelly covering, according to the 

 usual acceptation of the term, is more re- 

 stricted. It is possessed by a few ANNELIDA 

 (e. g. Serpula, Spirorbis, &c.), whose shelly 

 tubes so much resemble those of certain Mol- 

 lusks as to be readily confounded with them. 

 It is usually found, too, in the CIRRHOPODA, 

 (a class whose articulated character is now 

 quite settled) ; and it is generally present in 

 the CRUSTACEA, although it is only in the 

 larger and more highly developed forms of 

 this class, that the shell is consolidated by 

 mineral deposit, and really deserves the appel- 

 lation. 



The external configuration of the principal 

 varieties of shelly covering having been suf- 

 ficiently described under the several heads 

 above referred to, it is not requisite here to 

 revert to that subject ; our present purpose 

 being to give an account of the intimate struc- 

 ture of shell, on which an entirely new 

 light has been thrown by microscopical en- 

 quiries. The prevalent doctrine respecting 

 the nature of shell, as expressed even by the 

 most recent conchological writers, has been 

 that it is not only extravascular, but completely 

 inorganic, being composed of an exudation of 

 calcareous particles, cemented together by 

 animal glue. It may now, however, be stated 

 as an ascertained fact, that shell always pos- 

 sesses a more or less distinct organic struc- 

 ture* ; this being in some instances of the cha- 

 racter of that of the epidermis of higher animals, 

 but in others having more resemblance to that 

 of the dermis, or true skin. The nature of the 

 organic structure is so entirely different in the 

 Mollusca, Echinodermata, and Crustacea re- 

 spectively, that a separate description is re- 

 quired for each ; indeed, even in the subor- 

 dinate divisions of these groups very charac- 

 teristic diversities are frequently observable; so 

 that, as in the case of teeth, it is often possible 



* The idea that such would prove to be the case 

 was expressed by the author of this article in the 

 2d edition of his " Principles of General and Com- 

 parative Physiology" (published in 1841), as fol- 

 lows : " The dense calcareous shells of the Mollusca, 

 and the thinner jointed envelopes of the Crustacea, 

 have been commonly regarded as mere exudations 

 of stony matter, mixed with an animal glue secreted 

 from the membrane which answers to the true skin. 

 The hard axes and sheaths of the Polypifera, how- 

 ever, have been also regarded in the same light ; 

 and yet, as will hereafter appear, these are un- 

 questionably formed by the consolidation of what 

 was once living tissue. From the analogy which 

 the shells of Mollusca and Crustacea bear to the 

 epidermic appendages of higher animals, there would 

 seem reason to believe that the former, like the 

 latter, have their origin in cells, and that these are 

 afterwards hardened by the deposition of earthy 

 matter in their interior." P. 33. 



to determine the family, sometimes the genus, 

 and occasionally even the species, from the 

 inspection of a minute fragment of a shell, as 

 well fossil as recent ; whilst the degree of 

 correspondence or difference in the intimate 

 structure appears to be, in many groups, more 

 valuable than any other single character as a 

 basis for classification, because more indicative 

 of the general organisation of the animal. 

 Examples of both these applications will be 

 presently given. 



Mollusca. The shells of Mollusca may 

 always be regarded as epidermic in their cha- 

 racter; being formed upon the surface of the 

 mantle, which answers to the true skin of 

 other animals. As might be anticipated from 

 this description, they are essentially composed 

 of cells, consolidated by a deposit of carbonate 

 of lime in their interior ; but, as in other tissues, 

 we frequently find that the original cellular 

 organisation is obscured by subsequent changes, 

 and we sometimes lose all traces of it. We 

 shall first examine it in what may be consi- 

 dered its typical condition, which is most cha- 

 racteristically seen in certain bivalves. 



If a small portion be broken away from the 

 thin margin of the shell of any species of 

 Pinna, (this margin being composed of the 

 outer layer only, which projects beyond the 

 inner), and it be placed without any prepa- 

 ration under a low magnifying power, it pre- 

 sents on each of its surfaces, when viewed by 

 transmitted light, very much the appearance 

 of a honeycomb ; whilst at the broken edge 

 it exhibits an aspect which is evidently fibrous 

 to the eye, but which, when examined under the 

 rniscroscope with reflected light, resembles that 

 of an assemblage of basaltic columns. The 

 shell is thus seen to be composed of a vast 

 number of prisms, having a tolerably uniform 

 size, and usually presenting an approach to 

 the hexagonal shape. These are arranged 

 perpendicularly (or nearly so) to the surface 

 of the lamina of the shell ; so that its thick- 

 ness is formed by their length, and its two 

 surfaces by their extremities. A more satis- 

 factory view of these prisms is obtained by 

 grinding down a lamina until it possesses a 

 high degree of transparency ; and it is then 

 seen (fig. 407.) that the prisms themselves 

 appear to be composed of a very homogeneous 

 substance, but that they are separated by 

 definite and strongly-marked lines of division. 

 When such a lamina is submitted to the action of 

 dilute acid, so as to dissolve away the carbonate 

 of lime, a tolerably firm and consistent membrane 

 is left, which exhibits the prismatic structure 

 just as perfectly as did the original shell (Jig. 

 408.) ; the hexagonal divisions being evidently 

 the walls of cells resembling those of the pith 

 or bark of a plant, in which the cells are fre- 

 quently hexagonal prisms. In very thin natural 

 laminas, the nuclei of the cells can often be 

 plainly distinguished ; but we cannot expect to 

 find these, when the two ends of the cells (at 

 one of which they are generally situated) have 

 been removed by grinding. By making a 

 section of the shell perpendicularly to its 

 surface, we obtain a view of the prisms cut 



