FIELD BOOK OF PONDS AND STREAMS 



is no level of the shore waters which is not accessible to them. 

 They trail over the bottoms and along shore, climb the plant 

 stems, glide over the underside of the surface film (Fig. 34) 

 and swing down through the water on mucous threads. 

 They carry a hea\y load of shell and they can only move slow- 

 ly, but they leave no crevice unvisited and have literally 

 taken possession of every level of ponds and the shore waters 

 of lakes. 



In water deeper than six or eight feet the molluscan popu- 

 lation begins to fall off, the number of species becomes smaller 

 and where it is fifteen or twenty feet deep only a few kinds are 

 found. In his study of the mollusks of Lake Oneida, X. Y., 

 Baker found forty-six species in one to three foot shallows, 

 forty species in water three to six feet deep, and only eleven 

 species at a fifteen to eighteen foot depth. Shallows are the 

 usual haunts of snails yet there are records of pioneers like 

 LymncEa (Fig. 245), Planorhis (Fig. 249), and Valvata (Fig. 

 257) which have been found in Lake Michigan at depths of 

 eighty feet. 



Associates and enemies of mollusks. — 'Companies of small 

 snails and other animals seeking the same food can be found 

 beneath nearly every lily pad. Such gatherings usually 

 include a wheel-snail or two, Planorhis, a Physa, midge larvae 

 and bristle worms (Fig. 18). 



In spite of their shells, snails and mussels are eaten by nearly 

 all kinds of animals larger than themselves. Several leeches 

 such as Glossiplionia complanata (Fig. 118) live exclusively on 

 snails and worms. Other animals, certain dragonfly nymphs 

 (Epicordtdia), giant water-bugs, fishes, frogs, salamanders, 

 turtles, and even muskrats, eat them along with other food. 

 Piles of mussel shells are commonly seen near muskrat holes 

 (Fig. 238). Muskrats carry the mussels out of the muddy 

 shoal water onto land, piling them up near rocks or logs. 

 There they soon die and their shells gape open, exposing their 

 soft bodies to easy access. 



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