FIELD BOOK OF PONDS AND STREAMS 



only chance for life is in clamping their shells into the bodies, 

 gills, and fins of fishes (Fig. 263). They become imbedded 

 in the flesh, the skin grows over them, and they are protected 

 and nourished by the fish, being carried as actual parasites 

 for several weeks. 



The glochidia of one species of mussel will usually stay only 

 on certain species of fish and if they get onto the wrong one 

 they will drop off. This seems to be a proof of their extreme 

 sensitivity like that shown in their behavior toward fish blood. 



Pearly mussels, Family Unionidae. — These are large mussels 

 usually three or four inches long. Their shells are covered 

 with a dark colored horny periostracum and the inside is 

 pearly. The outer gill is used as a brood pouch. They are 

 abundant in the larger lakes and streams but are by no means 

 confined to them. 



This family includes all of the large mussels except a few 

 now grouped in the MargaritanidcB. The family Unionidce 

 has been split up into subfamilies containing many genera. 

 Representatives of only two genera are mentioned here. A 

 detailed treatment of the mussels will be found in Ward and 

 Whipple's "Fresh-Water Biology" and Baker's "Fresh 

 Water Mollusca of Wisconsin." 



Fig. 264. — Right side of Elliptio complanatus. 



Elliptic complanatus (formerly Unio complanatus). — The 



shells (Fig. 264) are thick, and oval or elongate, with smooth 

 or slightly corrugated surfaces. Their linings are nearly 

 always purplish and coppery. The breeding season is from 

 the last of April to the middle of May; the shells of the 

 glochidia are without hooks. 



320 



