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CHAPTER XVIII 



MOLLUSC A AND BRACHTOPODA 

 I 



THE various forms of * shell-fish,' with their * naked ' or shell-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 (1) the structure of the shell, which is most 

 interesting in the COXCHIFERA (or LAMELLIBRANCHIATA) and BRACHIO- 

 PODA, in both of which classes the shells are ' bivalve,' while the animals 

 differ from each other essentially in general plan of structure ; (2) 

 the structure of the tongue or palate of the GASTROPODA, 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 Gastropods present the greatest facilities. These three subjects, 

 therefore, will be first treated of systematically, and a few miscella- 

 neous facts of interest will be subjoined. 



Shells of Mollusca, These investments were formerly regarded 

 as mere inorganic exudations, composed of calcareous particles, 

 cemented together by animal glue ; microscopic examination, how- 

 ever, 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, taking as a type the group of Margaritacece, 

 which includes the Meleagrina or * 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 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 lamirise 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 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 segments of basaltic columns (fig. 696). This 

 outer layer is thus seen to be composed of a vast number of prisms, 

 having a tolerably uniform size, and usually presenting an approach 



