76 Descriptive Zoology. 



On the other hand, crayfishes often do very great damage by digging 

 holes in the dikes and levees along the lower Mississippi. These holes 

 gradually become enlarged, perhaps by muskrats, and may finally cause 

 the levee to give way, inundating vast tracts of land which are protected 

 by the dikes, and causing immense loss of property and sometimes of 

 life. 



Distribution of Crayfishes. Crayfishes are fairly common through- 

 out the United States, more especially in the central and southern por- 

 tions. They are usually more abundant in regions where there is plenty 

 of limestone, and are less abundant where granite rock prevails, as in 

 New England. They are to be found in Ireland and England and on 

 most of the continent of Europe, in Australia, New Zealand, Madagas- 

 car, and Japan. But they occur in very limited areas in Asia and South 

 America. In Africa none have ever been found. 



Origin of Crayfishes. It is supposed that the crayfishes are descend- 

 ants of marine crustaceans ; that some of these forms lived about the 

 mouths of rivers, gradually became accustomed to partly freshened 

 water, and in time to fresh water, and ascended the streams and entered 

 lakes. We do not know any living salt-water crustacean from which 

 the crayfish is supposed to have been derived. 



