322 BACKBONED ANIMALS. 
Fic. 348.—The beaver (Castor fiber), a gnawing water-animal, showing its 
dam and method of felling trees. 
is selected and dammed. Large trees, eighteen inches in diameter, 
are gnawed down (Fig. 348) and placed in position, and, if distant from 
the stream, a canal is built, often five hundred feet long, by which logs 
and food are floated to their homes. The logs are arranged against 
the current, curving up-stream, the interstices being filled with mud 
and other material. In working, the small matter is carried in the 
fore-paws, the webbed hinder ones and the tail being the organs of 
locomotion, and the latter perhaps used in moving logs and stones. 
The dam completed, the house is built under water, while burrows are 
made in the neighboring banks to be used as a last resort. The houses 
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