STRIJCTUEE AXD PnYSIOLOGY OF THE MOLLUSCA. 



29 



fusus antiquus it is cylindrical; in the pyramidelJidce it is 

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

 wliich are symmetrical when adult. 



The rudimentary shell of the nudihranchs is shed at an early 

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

 differs entirely from the shell of the crab 

 and other articulate animals, which is 

 periodically cast off and renewed. 



In the bivalves the embryonic shell forms 

 the umbo of each valve ; it is often very un- 

 like the after-growth, as in unio pidorum, 

 cydas henslowiana, and peden 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 vegetables derive 

 their elements from the mineral kingdom 

 (air, water, and the soil), and animals theirs ^^' ' ■^"' '^^' 

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

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

 the 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 varieties 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 shells is various and characteristic. Some, 

 when broken, present a dull lustre like marble or china, and 

 are termed porcelJanous ; others are pearly or nacreous; some 

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

 transluceyit. 



The nacreous shells are formed by alternate layers of very 

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



* Fip. 20. Cymha jirobo^tcidalis. Lam., from a very young specimen in tlie cabinet 

 of Hugji Cuming, Eeq., from Western Africa. 



