72 CONCHIFERA. 



attached in pairs. These branchiae are formed of a tissue of 

 small vessels arranged close together like the pipes of an organ. 

 At the sides of the mouth are four triangular thin leaflets, the 

 extremities of two lips. 



All the Conchifera have a testaceous covering of two prin- 

 cipal pieces, most of them of two alone. These pieces, named 

 valves, are opposed to one another, and constitute the proper shell 

 of the animal. The valves are united together near their base 

 by an elastic coriaceous or horny ligament, and the point of 

 union is called the hinge. This hinge is distinguished by teeth, 

 or protuberances and hollows, which lock into each other when 

 the shell is closed. When the valves are unequal or dissimilar 

 in size the shell is said to be inequivalve ; and when, on the 

 contrary, both resemble one another in their general form and 

 size they are said to be equivalve. Among the equivalve shells, 

 however, are found some which, when the shell is closed, have to- 

 wards their lateral extremities an opening or gape more or less 

 considerable. In those in which this opening is large, it has 

 been observed that the mantle of the animal is almost always unit- 

 ed before. 



The ligament of the valves is sometimes exterior and some- 

 times interior. In both cases it serves not only to fix the two por- 

 tions of the shell together, but to open them by its elasticity. 

 When this ligament is exterior, if the shell be closed, it is then 

 tense, the valves being held together by the contraction of the 

 internal muscle ; but if this muscle is relaxed, the elasticity of 

 the ligament alone separates the valves. When, on the contrary, 

 the ligament is interior, it is compressed when the shell is shut, 

 and the muscle exerts its power, but throws open the valves 

 when this power is relaxed. 



Though the Conchifera never crawl on a ventral disk or foot 

 like many of the Mollusca, yet some possess a muscular contrac- 

 tile organ, often compressed and lamelliform, which the animal 

 exserts or withdraws at will. This muscular part serves some 

 families as an organ of locomotion, by enabling them to execute 

 a sort of leap ; in others deprived of locomotion to attach their 

 tendinous threads or byssus to rocks or marine bodies. 



As the movements of this class are thus nearly reduced to 

 those of their muscular attachment to the shell and their mus- 

 cular cloak, these parts are much developed The thickness of 



