836 FRESH-WATER BIOLOGY 



also prefer these surroundings. These species are content with 

 hiding under other objects, and make holes only incidentally. 



2. Species living preferably in water with a rather strong current. 



(a) Species of the larger rivers. The subgenus Faxonius is typi- 

 cal for this habitat, and the location of its center of distribution in 

 the central basin with its large rivers expresses this. 



{b) Species Hving in small streams of the uplands. The repre- 

 sentatives of this habitat belong chiefly to the subgenus Bartonius, 

 and its distribution over the Appalachian Mountains and the Alle- 

 gheny and Cumberland Plateau clearly inchcates this. 



Of course, there are all transitions between habitats (a) and {b), 

 as many of the river species go well up into the head-waters, and 

 vice versa. Yet the original differentiation in the habitat of the 

 subgenera Faxonius and Bartonius is very evident. All these spe- 

 cies in running water are good burrowers, and they generally ex- 

 cavate holes under protecting stones, etc. In some of the species 

 from the mountain streams this faculty of burrowing is rather highly 

 developed, and leads us to the next ecological type. 



3. Burrowing species (" chimney builders ") . These species have 

 retired from the open water into the ground water, and one may 

 understand the origin of this peculiar habit by imagining that 

 forms in the small upland streams, with well-developed burrowing 

 faculties, were folrced, in periods of draught, when the streams in- 

 habited by them began to dry up, to dig down in the bed into the 

 gravel and mud, to reach the water. Or one may imagine, that 

 they ascended in the streams up to the sources, and went under 

 ground, where the water appears in the shape of springs. In a 

 number of species this tendency has been carried to an extreme, and 

 it is known that these Hve habitually under the surface of the 

 earth, in the ground-water, where they excavate more or less com- 

 plex systems of holes, burrows, or tunnels, which open upon the 

 surface in one or mor^ openings. These burrows are built by the 

 crayfish, by using the chelae in digging (hence the similarity of the 

 chelae in both sexes), and the material removed, mud, clay, etc., 

 is carried to the surface, where it is piled up around the mouth of 

 the burrow in irregular or regular piles, generally known by the 

 name of "mud chimneys." These burrows and chiefly the mud 



