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CHAPTER XIV. 



MOLLUSCOUS ANIMALS GENERALLY. 



The various forms of ' Shell-fish,' with, their naked' or Shell- 

 less allies, furnish a great abundance of objects of interest to the 

 Microscopist ; of which, however, the greater part may be grouped 

 under three heads ; — namely, (1) the structure of the Shell, which 

 is most interesting in the Conchifera or ' Bivalves ; ' (2) the 

 structure of the Tongue of the Gasteropoda, most of which have 

 1 Univalve ' shells, others, however, being ' naked ; ' (3) the 

 Developmental History of the Embryo, for the study of which 

 certain of the Gasteropods present the greatest facilities. — These 

 three subjects, therefore, will be first treated of systematically ; 

 and a few miscellaneous facts of interest will be subjoined. 



458. Shells of Mollusca. — These investments were formerly 

 regarded as mere Inorganic exudations, composed of Calcareous 

 particles, cemented together by Animal glue; Microscopic examina- 

 tion, however, has shown that they possess a definite structure, 

 and that this structure presents certain very remarkable variations 

 in some of the Natural Groups of which the Molluscous series is 

 composed. We shall first describe that which may be regarded as 

 the characteristic structure of the ordinary Bivalves ; taking as 

 a type the group of Margaritacece, which includes the ' Pearl- 

 oyster ' and its allies, the common Pinna ranking amongst the 

 latter. In all these Shells we readily distinguish the existence of 

 two distinct layers; an external, of a brownish -yellow colour; and 

 an internal, which has a pearly or 'nacreous' aspect, and is com- 

 monly of a lighter hue. — The structure of the outer layer may be 

 conveniently studied in the shell of Pinna, in which it commonly 

 projects beyond the inner, and there of ten forms laminae sufficiently 

 thin and transparent to exhibit its general characters without any 

 artificial reduction. If a small portion of such a lamina be exa- 

 mined with a low magnifying power by transmitted light, each of 

 its surfaces will present very much the appearance of a Honey- 

 comb ; whilst its broken edge exhibits an aspect which is evidently 

 fibrous to the eye, but which, when examined under the Micro- 

 scope with reflected light, resembles that of an assemblage of 



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