THE AQUATIC VERTEBRATES 1023 



rest plant feeders, though sometimes eating animal food. They 

 have been so reduced in numbers, in some places entirely exter- 

 minated, that they have become almost a negligible part of the 

 aquatic vertebrate fauna. Only the muskrat must be considered 

 as an ecological element in all eastern fresh waters. 



The muskrat is abundant along most of the eastern streams and 

 lakes. It is a shallow- water animal and affects its environment in 

 a specific way. It builds lodges of sod and cat-tail stalks, twigs 

 and vegetable debris. It gathers lily roots, on which it feeds, but 

 its most specific action is on various mussels. The muskrat lodge 

 is always surrounded by shells of dead bivalves, and at Winona 

 Lake it has been found by Headlee that the muskrat sets a bound- 

 ary to the shoreward migration of mussels as the soft bottom of 

 the pelagic area sets a limit to their migration toward deep water. 

 The activities of the muskrat are more restricted in winter than in 

 summer, but they do not hibernate. 



Beavers have disappeared from thickly-settled regions. They 

 are, in some of their habits, larger editions of the muskrat. They 

 build lodges not unlike those of the muskrat. They cut and 

 gather twigs and stems for food but the action for which beavers 

 are conspicuous, is the building of dams, and creating of ponds. 

 They thus add to the extent of 'the aquatic environment. 



The seal-like otter is no longer a part of the aquatic environment 

 in well settled parts of America. They are the most aquatic of 

 the fresh- water mammals. As swimmers, they are more expert than 

 fishes, which they catch and eat. They also prey upon muskrats 

 and aquatic birds. 



Of the star-nosed mole, Stone says: "The star-nosed mole is a 

 creature almost as well fitted for a partially aquatic life as the 

 otter and mink, and, as a matter of fact, does pass most of its time 

 about the water; pushing extensive tunnels through the black, 

 peaty soil of swamps and along the borders of little brooks and 

 ponds. The soft, black loam is thrown up in frequent heaps a 

 foot, more or less, in diameter; the opening of the burrow being 

 under the bank, and as often beneath the water as above. The 

 tunnel itself must frequently be flooded to the great discomfort 

 of its inmates. 



