PHYLUM MOLLUSCA 251 



After a parasitic life within the tissues of the fish of from three 

 to twelve weeks the young mussel is liberated and takes up a free 

 existence. 



In Unio the eggs are fertilized during the late spring and 

 summer, and the glochidia are discharged before the middle of 

 September. The glochidium of Unio is smajler than that of 

 Anodonta and is usually bookless. It does not as a rule be- 

 come permanently attached to the fins, operculum, or mouth 

 as in Anodonta, but usually lodges on the gill filaments of the 

 fish. 



One result of the parasitic habit of larval mussels is the dis- 

 persal of the species through the migrations of the fish. Only 

 in this way can we account for the rapid colonization of certain 

 streams by mussels, since the adult plows its way through the 

 muddy bottom very slowly. 



Economic Importance. Fresh-water mussels are of con- 

 siderable importance in certain parts of this country, especially 

 in Iowa and Illinois, because their shells are used extensively in 

 the manufacture of pearl buttons. Often, also, pearls of con- 

 siderable value are found in fresh-water bivalves. The de- 

 crease in the number of mussels in the Mississippi River and its 

 tributaries has led the United States Bureau of Fisheries to 

 investigate the possibility of artificially propagating them so as 

 to restock the depleted waters. It seems probable that this can 

 be done successfully. Mussels are instrumental in purifying 

 the water in which they live by using as food the organic particles 

 contained in it. 



2. CLASS I. AMPHINEURA 



The Amphineura are marine mollusks of wide distribu- 

 tion. Two rather distinct groups of animals belong to this 

 class. 



Order i. Polyplacophora. These are the chitons (Fig. 179, 

 A, B). They are characterized by a broad, flat foot (B, /), a 

 shell of eight transverse calcareous pieces (A), and a row of gills 



