Crayfishes are found in the fresh waters of the Northern and Southern Hemispheres ([Fig. 61]), but in each case they are practically confined to the temperate regions, and are absent from a broad intervening tropical zone. The Northern Crayfishes, forming the family Astacidæ (or Potamobiidæ) are distinguished, among other characters, by having a pair of appendages on the first abdominal somite, at least in the male sex; the Southern Crayfishes have no appendages on that somite, and for this and other reasons are regarded as constituting a distinct family—Parastacidæ. There is thus a general correspondence between the geographical distribution of the Crayfishes and the more important structural differences expressed in their classification. There can be no doubt that the two families have been derived from a common stock of marine lobster-like animals, and it is reasonable to suppose that two branches of this stock became independently adapted to a fresh-water habitat in the North and in the South, giving rise to the Astacidæ and the Parastacidæ respectively.

The distribution of the individual genera is, however, not so easy to understand. The species found in Europe all belong to the genus Astacus, which also penetrates into Asia as far as Turkestan and the basin of the River Obi.

Throughout the greater part of Asia no Crayfishes are found until we come to the Far East, where we find an isolated colony in the river-system of the Amur, in Korea, and in the north of Japan. These far eastern Crayfishes, however, differ so much from the typical species of Astacus that they are now placed in a subgenus (sometimes regarded as a distinct genus), Cambaroides. Curiously enough, the typical genus Astacus reappears again on the other side of the Pacific, where several species occur in that part of North America which lies west of the Rocky Mountains. East of the Rockies, again, numerous species are found belonging to a distinct genus, Cambarus, which ranges from Canada to Central America and Cuba, and this genus is allied in certain respects to the Cambaroides of Eastern Asia. If the systematic relations of these genera have been properly interpreted, it is by no means easy to understand in what way their present distribution has been brought about.

PLATE XX

THE MURRAY RIVER "LOBSTER," Astacopsis spinifer. NEW SOUTH WALES. (MUCH REDUCED)

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THE LAND CRAYFISH, Engæus cunicularis. TASMANIA (NATURAL SIZE)