STEUCTFEE AXD PHYSIOLOGY OF THE MOLLUSCA. 



29 



fusus antiquus it is cylindrical; in tlie pyramidellidce it is 

 oblique ; and it is spiral in carinaria, atlanta, and many limpets, 

 vrhich are symmetrical when adult. 



The rudimentary shell of the nudilrancTis is shed at an early 

 age, and never replaced. In this respect the molluscan shell 

 difiPers entirely from the shell of the crab 

 and other articulate animals, which is 

 periodicallj^ cast off and renewed. 



In the bivalyes the embryonic shell forms 

 the umho of each yalve ; it is often yery un- 

 like the after-growth, as in unio pidormn, 

 cyclas henslowiana, and pecfen pusio. In 

 attached shells, like the oyster and anomia, 

 the umbo frequently presents an exact imita- 

 tion of the surface to which the young shell 

 orignally adhered. 



Shells are composed of carbonate of lime, 

 with a small proportion of animal matter. 

 The source of this lime is to be looked for in 

 their food. Modern inquiries into organic 

 chemistry have shown that yegetables deriye 

 their elements from the mineral kingdom 

 (air, water, and the soil), and animals theirs 

 from the yegetable. The sea-weed filters the salt water, and 

 separates lime as well as organic elements ; and lime is one of 

 tho most abundant mineral matters in land plants. From this 

 source the mollusca obtain lime in abundance, and, indeed, 

 we find frequent instances of shells becoming unnaturally 

 thickened through the superabundance of this earth in their 

 systems. On the other hand, instances occur of thin and 

 delicate-shelled yarieties in still, deep water, or on clay bottoms; 

 whilst in those districts which are wholly destitute of lime, 

 like the Lizard in Cornwall, and similar tracts of magnesian- 

 silicate in Asia Minor, there are no mollusca. — {Forbes.) 



The texture of sheUs is yarious and characteristic. Some, 

 ^rhen broken, present a dull lustre like marble or china, and 

 are termed porceUanous ; others are pearly or nacreous; some 

 have a fibrous structure ; some are horny, and others glassy and 

 translucent. 



The nacreous shells are formed by alternate layers of veiy 

 thin membrane and carbonate of lime, but this alone does not 



Fig. 20. Cymha. 



* Fig. 20. Cymba proOoscidalis, Lam., from a very young specimen in the cabinet 

 of Hugh Cuming, Esq., from We!5teni Afiica. 



