270 COMPARATIVE ZOOLOGY. 



fluviatile or lacustrine, and a few are terrestrial air-breath- 

 ers. All bivalves, and nearly all univalves, are aquatic. 

 Each zone of depth in the sea has its particular species. 



CLASS I. Lamellibranchiata. 

 Lamellibranchs are all ordinary bivalves, as the Oyster 



and Clam. The shells differ from those of Brachiopods 

 in being placed on the right and left 

 sides of the body, so that the hinge is on 

 the back of the animal, and in being 

 unequilateral and equivalved. 138 The 

 umbo, or beak, is the point from which 

 the growth of the valve commences. 



FIG. 224. Pearl Oyster Both Brachiopods and Lamellibranchs 



(Meleagrina margariti- 



, one fourth nat- are headless ; but in the latter the mouth 



uralsize. Ceylon. pointg 



towards the anterior part. The length of the shell is 



measured from its anterior to its posterior margin, and its 



breadth from the dorsal side, where the 



hinge is, to the opposite, or ventral, edge. 



The valves are united to the animal by 



one muscle (as in the Oyster), or two (as 



in the Clam), and to each other by a 



hinge. In some species, as some fresh- 



water Mussels, the hinge is simply an 



elastic ligament, passing on the outside 



from one valve to the other just behind 



the beak, so that it is on the stretch when 



the valves are closed, and another placed FIG. 225. salt- water 



. , , ,, l . Mussel (Mytilus pel- 



between the edges ot the valves, so that inddus). Atlantic 

 it is squeezed as they shut, like the spring 

 in a watch-case. Such bivalves are said to be edentulous. 

 But in the majority, as the Clam, the valves also articulate 

 by interlocking parts called teeth. The valves are, there- 

 fore, opened by the ligaments, and closed by the muscles. 



