MOLLUSCOUS ANIMALS GENERALLY. 171 



CHAPTER XVI. 

 MOLLUSCOUS ANIMALS GENERALLY. 



561. THE various forms of ' Shell-fish/ with tReir 'naked' or shel- 

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

 scopist; of which, however, the greater part may be grouped under three 

 heads: namely, (I) the structure of the shell, which is most interesting in 

 the COSCHIFERA and BRACHIOPODA, in both of which classes the shells 

 are ( bivalve/ while the animals differ from each other essentially in gen- 

 eral plan of structure; (2) the structure of the tongue or palate of the 

 GASTEROPODA, most of which have '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. 



562. Shells of Mollusca. These investments were formerly regarded 

 as mere inorganic exudations, composed of calcareous particles, cemented 

 together by animal glue; Microscopic examination, however, has shown 

 that they possess a definite structure, and that this structure presents 

 certain very remarkable variations in some of the 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; tak- 

 ing as a type the group of Margaritacece, which includes the Avicula or 

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

 latter. In all these shells we readily distinguish the existence of two dis- 

 tinct 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. 



563. 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 laminas sufficiently thin and transparent to exhibit its 

 general characters without any artificial reduction. If a small portion 

 of such a lamina be examined with a low magnifying power by trans- 

 mitted light, each of its surfaces will present very much the appearance 

 of a honeycomb; whilst its broken edge exhibits an aspect which is evi- 

 dently fibrous to the eye, but which, when examined under the Microscope 

 with reflected light, resembles that of an assemblage of segments of 

 basaltic columns (Fig. 488, P). This outer layer is thus seen to be com- 

 posed 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 thickness is formed by their length, and its two 

 surfaces by their extremities. A more satisfactory view of these prisms 

 is obtained by grinding-down a lamina until it possesses a high degree of 



