GOO 



M O L L U S C A. 



Molluscs, ought to be considered at limited to hti'trs. These, in 

 V """Y~""*' some species, invest the surface regularly and closely 

 and may be observed on those which live on land, as well 

 as those which reside in water. In some cases these 

 hairs are as it were united, to form continuous crusts 

 or ridges. These hairs, as well as the cuticle, are li- 

 able to be worn off, and in some places can seldom be 

 perceived, unless in early age. 



Shell. The most important appendix of the skin appears 



to be skill. This part is easily preserved, exhibits fine 

 forms and beautiful colours, and has long occupied the 

 attention of the conchologist. The matter of the shell 

 is secreted by the corium, and the form which it as- 

 sumes is regulated by the body oi' the animal. It is 

 coeval with the existence of the animal, antl appears 

 previous to the exclusion from the egg ; nor can it be 

 dispensed with during the continuance of existence. 

 The solid matter of the shell consists of carbonate of 

 lime, with a small portion of animal matter, resembling 

 coagulated albumen. 



The mouth of the shell is extended by the applica- 

 tion of fresh layers of the shelly matter to the margin, 

 and its thickness is increased by a coating on the inner 

 surface. These asbertions are abundantly confirmed 

 by the observations of Reaumur, (Memoir es de I'Aca- 

 dearie des Sciences, 1709,) \vho?e accurate experiments 

 have greatly contributed to the elucidation of concho- 

 logy. If a hole is made in the shell of a snail, and a 

 piece of skin glued to the inner margin, so as to cover 

 the opening, the shelly matter does not ooze out from 

 the broken surface, so as to cover the external surface 

 of the skin, but it forms a coating on its inner sur- 

 face, thus proving it to have exuded from the body of 

 the animal. When a considerable part of the oral part 

 of the spire of a snail is broken off, and a piece of skin 

 glued to the inner margin, and reflected outwardly 

 and fixed on the body of the shell, the spire is again 

 renewed, and the matter added to the inner surface, 

 thus leaving the skin interposed between the new form- 

 ed portion and the fractured edge. Similar experi- 

 ments, repeated on a variety of shells, both univalve 

 and bivalve, by different naturalists, leave no room to 

 doubt that shells increase in size by the juxtaposi- 

 tion of shelly matter from the common integuments. 



Each calcareous layer is more or less intimately mixed 

 and enveloped in the animal matter, which we have al- 

 ready alluded to; so that the different layers of successive 

 growth may, by various processes, be distinctly exhi- 

 bited. If the shell is exposed for a short time in the fire, 

 the animal matter becomes charred, and its black colour, 

 contrasted with the -white earthy matter, indicates the 

 different strata : in the same manner as the ivory and 

 enamel of a tooth ran be distinguished when subject- 

 ed to similar treatment. The same satisfactory results 

 may be obtained by a different process. If the shell 

 be steeped in weak muriatic acid, the earthy matter 

 will be dissolved, and the flakes of albumen will re- 

 main as the frame-work of the edifice. 



The layers of growth may often be distinguished on 

 the surface of the shell, in the form of striae, or ridges 

 more or less elevated, but parallel to the margin of t' e 

 aperture. I Other inequalities may likewise be observ- 

 ed on the surface, at right angles to the layers of 

 growth, such as ridges, knobs, and spines. These 

 last derive their origin from the inequalities of the skin 

 on which they have been moulded. 



In some univalve shells, the layers of growth paral- 

 lel to the opening cannot be discerned ; when exposed 

 in ithe 1 fire, there is little darkening of colour ; and 

 when dissolved in acids, but a feeble trace ol animal 



matter remains. In the fire, these shells crack in vari- Mollusc*. 

 ous directions, but exhibit no trace of a scaly struc- "" T " 

 ture. By a careful management with the file, the shell 

 may be separated into a central layer contiguous to the 

 skin, and a peripheral layer, both similar in structure, 

 though frequently differing in colour. The shells ex. 

 hibiting such characters have been termed Porcellane- 

 ous, from their dense structure, and the fine polish 

 which their surface presents. The formation of shells 

 of this kind must he executed in a different manner 

 from those of the first kind which we have noticed. 



If we attend to the form of a young shell of the ge- 

 nus Cyprrea of Linnanis, we may perceive that an ad- 

 dition of shelly matter to the margin of the aperture, 

 in the manner in which it is applied in other shells, 

 would not enlarge the cavity, but completely close the 

 aperture. The increase of the shell, (accompanied with 

 a corresponding increase of its inhabitant,) must take 

 place either by absorption of the accumulated sheily mat- 

 ter of the mouth, and an elongation in the direction of 

 the greatest curvature of the shell ; or the old sht II .must 

 be thrown off, and a new one produced suited to the size 

 of the animal. The former supposition has not been en- 

 tertained, the latter is DOW generally received by natural- 

 ists. The inner coat of s^ch shells appears to be a trans- 

 udation from the body of the animal, the outer one ap- 

 plied to the surface by the loose reflected lobes of the 

 cloak. In many other shells, portions of more compact 

 matter than the other parts may be observed, spread on 

 the pillar, and applied to the margin of the mouth by a 

 similar process. Mr. Platt, in support of Reaumur's 

 opinion, that shells are formed by juxtaposition, against 

 the objections -of Mr. I'oupart, (Phil. Trans, vol. liv. 

 p. 43.) erroneously considers the different sizes of the 

 Cypreas as depending on the thickness of the shell in- 

 creasing according to age, without admitting a corres- 

 ponding increase of the dimensions of th contained 

 animal, or cavity for its reception. 



The shells of the first kiiid which we have noticed, 

 from the manner in which they are formed, of cones or 

 layers applied to the inner edge of the margin, and ex. 

 tending beyond it, have an imbricated structure. Those 

 of the second kind, consisting of layers regularly super- 

 imposed, have const qmntly a laminated structure; but 

 between the two kinds there are numerous intermediate 

 links, formed by a combination of the two piocesses. 



In some cases, the hard parts of the skin are not en- 

 titled to the appellation of shell, but may rather be con. 

 sidered as horn. Such are the coverings of the mandi- 

 bles of the Cuttle fish, the branchial lid of the Aplysia, 

 and the operculum of the Welk. The two last appen- 

 dices, however, though horny in some species are 

 shelly in others. 



The position of the shell with respect to the constituent 

 layers of the integuments, exhibits very remarkable dif- 

 ferences. In some it appears instead of a cuticle, or 

 at least external membrane investing it. In gene- 

 ral, however, it occurs between the cuticle and the 

 skin, a position which induces Cuvier (Lee. d' An. Comp. 

 xiv. 11.) to consider it as analogous to the mucous web 

 of the vertebral anm'als. Its intimate connection with 

 the muscular system of the animal, and the protection 

 which it yields, seem adverse to such a conclusion. In 

 many species the testaceous substance occurs in folds 

 of the corium, or inserted in its substance. In this po- 

 sition it never acquires the solid texture which shells 

 exposed, or covered only by the cuticle, exhibit. Those 

 which are thus concealed are in gnu-nil white; 

 those which are more exposed are f- qiifiitly co 

 loured. The colouring, however, does not depend 



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