Fig. 60—Asellus aquaticus, Female. × 4. (After Sars.)

[View larger image]

In Australia and New Zealand the Isopoda are represented in fresh waters by a very peculiar group of species, forming the suborder Phreatoicidea, which have more the aspect of Amphipods than of Isopods, since the body is more or less flattened from side to side, instead of from above downwards.

With regard to the mode of distribution of the fresh-water Isopoda, there is the same difficulty as in the case of the Amphipoda, for the eggs are carried in a brood-pouch, and do not seem to be in any way protected against drought. It is no doubt in consequence of this that the fresh-water species and genera of both Amphipoda and Isopoda, though widely distributed, do not have the world-wide range of many of the more minute Crustacea described above.

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.

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)

[View larger image]