The common Crayfish, _Astacus_ (or _Potamobius_) _pallipes_, is the only truly fresh-water Decapod found in England, although a small Prawn, _Palæmonetes varians_, which usually inhabits brackish water, may occasionally be found in places where the water is practically fresh. The structure of the Crayfish is very similar to that of the Lobster, but, as already mentioned, it differs in its mode of development, having no free-swimming larval stage. From its size, and from the fact that the eggs are carried by the female, the Crayfish cannot be transported from one locality to another by the agencies which distribute the smaller fresh-water Crustacea. On the other hand, the adult animals can live out of the water for days, or even weeks, if they are kept moist, and the English species is stated to leave the water occasionally, and to make short excursions on land. Many species found in foreign countries are still more truly amphibious in their habits. It is clear, however, that the means of dispersal of the Crayfishes are very limited, and on this account the problems connected with their geographical distribution are of great interest. An admirable discussion of the subject will be found in Professor Huxley's book on the Crayfish, and the conclusions reached by him have hardly been modified by thirty years of subsequent research. Only a very brief outline can be attempted here. [Illustration: FIG. 61--MAP SHOWING THE DISTRIBUTION OF CRAYFISHES. (Partly after Ortmann.) The dotted areas are those occupied by the Northern Crayfishes (family Astacidæ). The black patches mark the areas inhabited by the Southern Crayfishes (family Parastacidæ)] 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.