272 MOLLUSCA. 



body, which contains the liver and viscera, is placed between the 

 two laminae of .the mantle; forwards, and still between these laminae 

 are the four branchial leaflets, transversely and regularly striated by 

 the vessels : the mouth is at one extremity, and the heart towards 

 the back; the foot, when it exists, is inserted between the four bran- 

 chias. On the sides of the mouth are four triangular leaflets, which 

 are the extremities of the two lips, and serve as tentacula. The 

 foot is a mere fleshy mass, the motions of which are effected by a 

 mechanism analogous to that which acts on the tongue of the Mam- 

 malia. Its muscles are attached to the bottom of the valves of the 

 shell. Other muscles, which sometimes form one mass and some- 

 times two, cross transversely from one valve to the other to keep 

 them closed, but when the animal relaxes these muscles, an elastic 

 ligament placed behind the hinge opens the valves by its contraction. 



A considerable number of bivalves are provided with what is 

 termed a byssus, or a bundle of threads more or less slender, which 

 issues from the base of the foot, and by which the animal adheres 

 to various bodies. It uses its foot to direct the threads and to 

 agglutinate their extremities; it even reproduces them when cut, 

 but the nature of the production is not thoroughly ascertained. 

 Reaumur considered these threads as a secretion, spun and drawn 

 from the sulcus of the foot; Poli thinks they are mere prolongations 

 of tendinous fibres. 



The shell consists essentially of two pieces, called valves, to which 

 in certain genera are added others, connected by a hinge that is 

 sometimes simple and sometimes composed of a greater or smaller 

 number of teeth and plates, which are received into corresponding 

 cavities. 



There is usually a projecting part near the hinge called the summit. 



Most of these shells fit closely when the animal approximates 

 them, but there are several which exhibit gaping portions either be- 

 fore or at the extremities. 



FAMILY I. 



OSTRACEA. 



The mantle is open, without tubes or any particular aperture. 

 The foot is either wanting in these Mollusca or is small ; they are 



