STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 



37 



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 umho of each valve ; it is often very unlike 

 the after-growth, as in unio pictorum, cyclas 

 hendowiana and pecten push. In attached 

 shells like the oyster and anomia the umbo fre- 

 quently presents an exact imitation of the sur- 

 face to which the young shell originally adhered. 



Shells are composed of carbonate of lime, 

 with a small proportion of animal matter. 

 The source of this lime is to be looked Fig. 20. Cymha* 

 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 whoUy 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 cliina, and are 

 termed porcellanous ; others are pearly or nacreous ; some have 

 Bl fibrous structui'e ; some are horny, and others glassy and trans- 

 lucent. 



* Fig. 20. Cyrnba proboscidalis. Lam., from a very young specimen in 

 the cabinet of Hugh Cuming, Esq., from Western Africa. 



