45 



PART III. 



CONCHIFERA OK, BIVALVES. 



The Mollusca inhabiting bivalve shells are 

 chiefly distinguished from the other classes by 

 the absence of a visible head or neck, and the 

 consequent deprivation of the organs of sight 

 and hearing ; they possess a mouth, but it is a 

 mere opening in the body, without jaws, teeth, 

 or hard parts. The branchiae are large, placed 

 on each side, between the body and the mantle. 

 The lobes of the mantle are fringed round the 

 edge with numerous filaments, very sensitive, 

 and irritable to the touch, and which are in con- 

 stant activity, drawing a current of water within 

 for the capture of their prey, which is supposed 

 to consist of small particles of vegetable and 

 animal substances. None of the genera are ter- 

 restrial, their construction not affording them 

 sufficient powers of locomotion for finding their 

 food on land, and confining them to the water, 

 whether salt or fresh, or to the sands on the 

 coasts. 



Bivalves are separated into three orders : — 



