FRESH-WATER MUSSEL 



215 



marsupium and to be carried out of the dorsal siphon by the currents of 

 water flowing through the body. Since the unios usually live in running 

 water, their glochidia are swept along by the current until they are carried 

 into the mouths of fish with the water which the latter use in respiration. 

 As they pass the gills of the fish they attach themselves to the gill fila- 

 ments, clinging by the valves of the shell and also, perhaps, attached by 

 the larval thread, which penetrates the tissues. The closure of the valves 

 seems to be due to chemical stimulation by salts escaping from the tissues 

 of the fish. On the gills of the fish the larvae live a parasitic existence 



Fig. 122. — Shell of a unio, Quadrula undulata (Barnes). From the Grand River, 

 Michigan. For comparison wnth the anodonta type, Figs. 116 and 117. This is one of 

 the species most sought for button making. X %. 



until development is completed and they are ready to metamorphose into 

 small adults. After this has occurred they escape from the fish, fall to 

 the bottom, and begin an independent Hfe. 



256. Other Fresh-water Mussels. — A large number of species of 

 mussels, belonging to several genera, may usually be found in any portion 

 of this country. They differ markedly in size and shape and in the details 

 of structure. They may be all grouped under two types — the stream 

 type, known in a general way as unios, and the quiet-water type, known 

 as anodontas. The former (Fig. 122), usually found in running water, 

 are generally stationary, have thick shells with prominent hinge teeth, 

 and the glochidia develop mostly in the gills of fishes, though they may 

 also be attached to external surfaces. The anodonta type (Figs. 116 and 

 117), on the other hand, includes forms that live in still water, moving 



