4 



POPULAR CONCHOLOGY. 



less completely, and from which they are able 

 to exude a liquid, which, on exposure to the air 

 or water, hardens into shell, and thus most of 

 them are enabled to cover themselves with a 

 secure, and at the same time commodious and 

 beautiful habitation, into which they can, by 

 the aid of strong contractile muscles, retire far 

 enough to escape from danger when it ap- 

 proaches. This outer skin is now usually called 

 the mantle, and is common to most species of 

 mollusca. As nearly all these animals live en- 

 tirely in the water, from which they derive 

 their nutriment, and which they breathe by the 

 aid of branchice*, resembling the gills of fish, it 

 is necessary that a means should be provided 

 for the admission of that fluid to the interior of 

 the body, and in order to effect this, certain 

 openings occur in the mantle, through which 

 the water passes, and by which also the head 

 and foot, when these limbs exist, are put forth 

 and drawn back at the will of the animal. 



Mollusca do not all possess heads, one entire 

 class being quite destitute ; when they do exist, 



* The fringed appendages in the interior of the oyster 

 are the branchiae of that Mollusc. 



