MOLLUSCA. 



159 



fering in size, shape, and arrangement, in different families, 

 and containing secreted calcareous matter. There seems reason 

 to believe, "that this membrane was, at one time, a constituent 

 part of the mantle of the Mollusk ;" and Dr. Carpenter regards 

 the cells as "the real agents in the production of shell, it being 

 their office to secrete into their own cavities the carbonate of 

 lime supplied by the fluids of the animal."* 



The deposition of the colouring matter is the province of 

 glands situated on the margin of the cloak or collar ; and in 

 many instances we are able to trace an agreement in the 

 pattern or tracings on the shell and the arrangement of the 

 colours in the secreting organ. Thus, in the banded Snail, 

 there are as many coloured spots on the edge of the collar as 

 there are zones on the shell; and if a part of the margin of 

 the shell be cut away, the piece reproduced is brown opposite 

 to the dark portion of the collar, but in other parts yellow. 



The changes of form which shells undergo, as they approach 

 maturity, is sometimes so great, that the full- grown. specimen 

 is altogether different from the appearance presented by the 

 same shell in its immature state. Of 

 this the common Leg-of-mutton Shell 

 (Aporrhais pes pdicani, Fig. 152) of 

 our shores, and the beautiful tribe of 

 Cypreas (Fig. 153), furnish familiar 

 examples. AVe have reason to believe 

 that there is, in all cases, an effort on 

 the part of the animal to accommodate 

 the form of its mansion to the changes 

 in the form or dimensions of its body. 

 Professor Owcnf has stated that an 

 oyster kept without food will frequently 

 expend its last energies in secreting a 

 new layer, "at a distance from the old 

 internal surface of the concave valve, 



corresponding to the diminution of bulk which it has experi- 

 enced during its fast, and thus adapt its inflexible outward 

 case to its shrunken body." 



It has been justly remarked, that the beauty of shells was 

 for ages exerting an influence injurious to the study of 



* On the Microscopic Structure of Shells. Report of British Associa- 

 tion, 1844. 



f Proceedings Zoological Society, No. liv. 

 PART i. II 2 



Fig. 152. APORRHAIS. 



