222 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



are inclosed in a shell composed of two parts or valves they are 

 commonly referred to as bivalves. 



The class Gastropoda (Gr. gaster, stomach; pous, foot) in- 

 cludes the snail, slugs, periwinkles and many other molluscs 

 that are either naked or furnished with a shell composed of a 

 single piece. 



The Cephalopoda (Gr. kephale, head; pous, foot) include 

 squids, octopi, cuttlefish and the nautilus. 



The members of the classes Amphineura (Gr. amphi, around; 

 neuron, nerve) and Scaphopoda (Gr. skaphe, hollow; pous, foot) 

 are much less common and are of little or no economic impor- 

 tance. The first includes the chitons, which have segmented 

 shells and are fairly common on rocks on the California coast. 

 The tooth-shells sometimes found along the northern sea 

 beaches belong with the peculiar somewhat worm-like mem- 

 bers of the class Scaphopoda. 



CLASS PELECYPODA 



The Mussels. They are many genera and species of fresh- 

 water mussels, and they are to be found in suitable streams 

 and lakes in almost all parts of the United States. Their life 

 history and importance have just been discussed. The salt- 

 water mussels differ from those in fresh water in several 

 respects, the most noticeable of which is in shape and in the 

 presence of a number of fine tough threads, the bysstis, which 

 serve to attach the mussels to rocks or other substances on 

 which they are growing. These mussels often occur in great 

 masses over the rocks and piles or on tide flats wherever they 

 can find a place to attach themselves. They are often serious 

 pests on oyster beds, occurring in such numbers as to smother 

 the oysters or starve them by taking a large part of the food 

 that would otherwise go to the oysters. The salt-water mussels 

 are often used for food, and can advisably be thus used more 

 than they are at present. They occur in abundance on both 

 Atlantic and Pacific coasts of our country, and are easily 

 gathered at low tide. 



Clams. In most of the clams the portion of the mantle that 

 forms the siphons in the mussels is especially developed and 



