MOLLUSCA 



125 



CLASS i. Pelecypoda 



These mollusks, formerly called 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. 25 The umbo, or beak, is 

 the point from which the growth of the 

 valve commences. Both brachiopods 

 and pelecypods are headless; but in FIG. 8 4 . -Pearl Oyster 



. , . (Meleagrina mar gar i- 



tne latter the mouth points the same u/eray, one fourth nat- 

 way as the umbo, i.e., toward the uralsize " Ceylon ' 

 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 liga- 

 ment, passing on the outside from one 

 valve to the other just behind the beak, 

 so that it is stretched when the valves 

 are closed. Another is placed between 

 the edges of the valves, so that 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 and the fresh- 

 water Unio, the valves also articulate 

 by interlock-parts called teeth. The 

 valves are, therefore, opened by the 

 ligaments, and closed by the muscles. 

 The shell is secreted by the mantle. 



FIG. 85. Salt - water 

 Mussel (Mytilus pel- 

 lucidus). Atlantic 

 coasts. 



