HIGHER CRUSTACEANS (MALACOSTRACA) 835 



and may take place in all three directions: forward, backward, 

 and sideward. More rarely the crayfishes move by swimming, and 

 chiefly so when alarmed and trying to escape; this swimming is 

 always backward, and is effected by quickly repeated strokes of 

 the abdomen. This kind of locomotion, however, is kept up only 

 for short distances. 



With regard to the habitat, not much detail is known in the iso- 

 pods and amphipods. They seem to prefer more quiet bodies of 

 water, small streams and springs, to the larger rivers. Some of 

 them are not very particular as to their habitat, and consequently 

 possess a very wide geographical distribution, while others are very 

 restricted, possibly on account of special habitat preferences. The 

 only Mysidacean found in North America (Mysis relicta) inhabits 

 the Great Lakes to a considerable depth (as do two species of the 

 Amphipod-genus Pontoporeia) . The genus Palaemon of the Deca- 

 pods is known only from our largest rivers (Mississippi and Ohio). 



In the genus Cambarus, very complex conditions are observed 

 and the different species differ considerably in their ecology. Al- 

 though they all need water for their existence, it is a general rule 

 that all crayfishes are able to leave the water temporarily, and 

 some may stay out of the water for a considerable time, and do so 

 habitually. Of course, in order to moisten their gills, they always 

 have to return to the water. 



In the water, the crayfishes try to hide, either under rocks, logs, 

 water weeds, etc., or they construct artificial hiding places (holes 

 and burrows). The latter tendency is, as will be seen, especially 

 developed in certain ecological groups. In connection with this 

 tendency to hide probably is the fact that the crayfishes seem to be 

 more or less nocturnal. 



With regard to their ecological preferences, different types have 

 been distinguished in the genus Cambarus. These are the fol- 

 lowing : 



i. Species living in quiet waters: slowly running, large rivers, 

 ponds, lakes. To this group belongs chiefly the subgenus Cam- 

 barus, and its distribution over the coastal plains and the interior 

 basin expresses this ecological habit, since here such conditions are 

 pre-eminently found. But certain species of the subgenus Faxonius 



