8 4 6 



FRESH-WATER BIOLOGY 



34 (29) Body subcylindrical in its anterior part, abdomen depressed. Ros- 

 trum depressed. Pleura of second abdominal segment not 

 overlapping those in front. First three pairs of peraeopods 

 chelate, the first pair much larger than the others. 



Family POTAMOBIIDAE . . 35 



An exclusive fresh-water family of old age, and the most important group of higher crusta- 

 ceans in the fresh waters of North America. Its general distribution includes Europe, north- 

 eastern Asia, North and Central America. In the United States two genera are found: one, 

 containing a few species, is believed to be identical with the European genus (Potamobius) ; the 

 other (Cambarus) is restricted to America, and has very many species. The differences of 

 these genera are found chiefly in the sexual apparatus. 



In the southern hemisphere, in Australia, New Zealand, South America, and Madagascar 

 this family is represented by an allied one, Paras tacidae while in the tropical belt similar 

 forms are missing. This peculiar distribution has given origin to much speculation, and 

 chiefly the close affinity of the southern forms has been introduced as evidence for the former 

 connection of the southern continents. 



Through Huxley's book (1880) this family has become a standard group for biological study. 



35 (36) Male copulatory organs rather simple. Peraeopods of male without 

 hooks on the ischiopodite. Female without receptaculum 

 seminis. A pleurobranchia present on the last thoracic 

 somite Potamobius Leach. 



This is the genus which includes the Euro- 

 pean crayfishes, frequently, but incorrectly, 

 called Astacus. It possesses five species in 

 North America, the range of which is on the 

 western Pacific slope, from California to 

 British Columbia. One species (P. gambeli 

 Girard), has crossed the continental divide 

 in the region of Yellowstone Park, and is 

 found on both sides in the drainages of the 

 upper Columbia River and of the upper 

 Missouri. 



The European species (about six) have 

 frequently been subjects of systematic, ana- 

 tomical, biological, and embryological 

 studies. The best known species is the 

 common crayfish of Central Europe, Pota- 

 mobius astacus (Linnaeus). 



FlG. 1313. Potamobius trowbridgei Stimpson. 

 X*. (After Hagen.) 



A species found abundantly in the drain- 

 age of the lower Columbia River in Wash- 

 ington and Oregon, and of considerable 

 economic value. 



