THE CLAM AND OTHER BIVALVES 175 



Relation to Environment. The distribution of Unio com- 

 planata in the bed of a river is determined somewhat defi- 

 nitely by the position of the strongest current. It is well 

 known that in a crooked river the main current does not 

 follow the middle line of the stream, but sweeps from bank 

 to bank diagonally across the middle. The greater amount 

 of food is carried along by the swifter water, and hence, 

 unless the stream is too swift, the mussels find it advanta- 

 geous to range themselves along the line of the most abun- 

 dant food-supply. 



Mussels are considered by some to be of great service to 

 man and other animals because of their habit of devouring 

 the decaying organic substances so abundant in rivers which 

 flow past large cities. There is good reason for believing, 

 however, that sewage is not the natural food of mussels, and 

 that they will flourish better in water of natural purity. In 

 fact, it has been positively determined that mussels do not 

 live near the mouth of a large sewer. 



DEFINITION OF PELECYPODA 



The four animals described in this chapter are representa- 

 tives of a class called by various authors Lamellibranch'ia, 

 Aceph'ala, or Pelecyp'oda (Gr. pelekys, hatchet ; pom (pod), 

 foot). They are also called bivalves because the shell is 

 in two pieces. Lamellibranchia signifies that the animals 

 have gills (branchige) which are thin, like plates (lamellae). 

 Acephala means " without a head." 



Pelecypoda are usually bilaterally symmetrical animals with 

 an external skeleton composed of two more or less nearly 

 equal valves. They have no internal skeleton. The body is 

 without a head and is not divided into somites. There are 

 no segmented appendages. The mantle-folds surround the 

 body proper and secrete the shell substance. Locomotion is 



