CHAPTER XIY. 



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 "Univalve" shells, others, however, being "naked;" and 



(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. 



336. Shells of Mollusca. These investments were formerly re- 

 garded as mere inorganic exudations, composed of calcareous 

 particles cemented together by animal glue ; Microscopic exa- 

 mination, however, has shown that they possess a distinctly or- 

 ganic structure ; and this structure presents certain very remark- 

 able variations, in some of the natural groups of which the Mol- 

 luscous 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 in- 

 cludes the "Pearl-oyster" and its allies, the common Pinna 

 ranking amongst the latter. In all these shells, we readily dis- 

 tinguish the existence of two distinct layers ; an external, of a 

 brownish-yellow color; and an internal, which has a pearly or 

 " nacreous" aspect, and is commonly 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 often forms laminae sufficiently thin and transparent 

 to exhibit the general nature of its organization without any 

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

 mined with a low magnifying power, even without any prepara- 

 tion by transmitted light, each of its surfaces will present very 

 much the appearance of a honeycomb ; whilst its broken edge 

 exhibits an aspect which is evidently fibrous to the eye, but 

 which, when examined under the microscope with reflected light, 

 resembles that of an assemblage of basaltic columns (Fig. 334, p). 



