INTRODUCTION. xvii 



and are enveloped in a muscular covering called the 

 mantle. Their nervous system is broken up into several 

 pieces, which are disposed in various parts of the 

 body and connected together by stout nerve-cords. 



Most molluscs are provided with a shell which is 

 formed by a secretion of the mantle, composed chiefly 

 of carbonate of lime with an admixture of more or 

 less animal matter. 



The Mollusca are divided into ^^^>^^/^2/j (headless) 

 and Cephalic {)\-d.vmg2i head). The shells of the former 

 are Bivalve^ those of the latter Univalve. 



Acephala. — All the Acephala are aquatic ; some of 

 them inhabit fresh water, but by far the greater 

 number are marine, and of these, the oyster and 

 cockle will serve as examples. 



They have no distinct head ; the moiith is seated 

 within the folds of the mantle, and consists of a slit 

 or opening with two flattened lips. The mantle which 

 envelops the body consists of two parts or lobes united 

 at the back. Gwyn Jeffreys admirably describes it as 

 like " the cover of a book ; " it is attached to the 

 inner margin of each valve of the shell by a series of 

 muscles ; in some cases it is open on all sides (except 

 the back) ; in others it is more or less closed in front, 

 but there is always an opening sufficiently large to 

 admit of the passage of the foot, which is tongue- 

 shaped, flexible, and often capable of being consider- 

 ably extended beyond the shell. But in the case of 

 the Brachiopoda (a very remarkable class of marine 

 headless molluscs) the animals have no foot, they are 

 sessile, being attached to stones, seaweeds, or other 

 objects, either by a foot-stalk or the outer surface of 



b 



