THE CLAM AND OTHER BIVALVES 171 



would be different from the actual performance. The attitude 

 of a swimming scallop is portrayed in the upper left-hand part 

 of the illustration. In the act of swimming the valves open 

 and close quickly, by the alternate action of the hinge-liga- 

 ment and the large adductor muscle. On closing, the valves 

 catch a quantity of water between the mantle-folds. The 

 water escapes under pressure from within, through a round 

 opening at either end of the straight flange of the hinge. 

 The resulting action of these jets of water backward, against 

 the body of water outside, is to force the larger and broader 

 end of the animal forward. Locomotion is not in a direct line, 

 but over a zigzag course. When the scallop ceases swimming 

 it immediately falls to the bottom. The animal is not what 

 one would call a skillful swimmer, although its movements 

 are very interesting to observe. 



THE FRESH-WATER MUSSEL 



Habitat and Distribution. In fresh waters generally, wher- 

 ever sufficient carbonate of lime is carried in solution, one 

 may find mussels living nearly covered in sand and mud. 

 The species represented in Fig. 88, U'nio complana'ta, is 

 distributed in the rivers and brooks of the entire country. 



Comparison with Other Forms. The valves of the mussel are 

 equal, like those of the clam and the scallop. The valves of 

 the .fresh- water species are held together by a hinge-ligament, 

 aided by two pearl-covered ridges running parallel and fitting 

 into grooves. The mantle-folds are not united, as they are in 

 the clam. At two adjoining places in the posterior region the 

 rim of the mantle-folds is fringed with short tentacles. Dorsal 

 and ventral tubes are formed by the meeting of opposite edges 

 at the places where the tentacles occur. Food and oxygen are 

 carried in by the ventral tube, and undigested substances are 

 carried out by the dorsal tube. We may speak of the siphon 



