It will be observed that while the two forms, _A. torrentium_ and _A. nobilis_, are intermixed over a large part of Central Europe, _A. torrentium_ has a wider north-westward, south-westward, and south-eastward extension, being the sole occupant of Britain, and apparently of the greater part of Spain and of Greece. On the other hand, in the northern and eastern parts of Central Europe, _A. nobilis_ appears to exist alone.
Further to the east, a new form, _Astacus leptodactylus_ (fig. 75), makes its appearance. Whether _A. leptodactylus_ exists in the upper waters of the Danube, does not appear, but in the lower Danube and in the Theiss it is the dominant, if not the exclusive, crayfish. From hence it extends through all the rivers which flow into the Black, Azov, and Caspian Seas, from Bessarabia and Podolia on the west, to the Ural mountains on the east. In fact, the natural habitat of this crayfish appears to be the watershed of the Pontocaspian area, excluding that part of the Black Sea which lies southward of the Caucasus on the one hand, and of the mouths of the Danube on the other.[25]
[25] These statements rest on the authority of Kessler and Gerstfeldt, in their memoirs already cited.
It is a remarkable circumstance that this crayfish not only thrives in the brackish waters of the estuaries of the rivers which debouche into the Black Sea and the Sea of Azov, but that it is found even in the salter { 302} southern parts of the Caspian, in which it lives at considerable depths.
[Illustration: FIG. 75.—Astacus leptodactylus (after Rathke, 1/3 nat. size).]
In the north, _Astacus leptodactylus_ is met with in the rivers which flow into the White Sea, as well as in many streams and lakes about the Gulf of Finland. But it has probably been introduced into these streams by the canals which have been constructed to connect the basin of the Volga with the rivers which flow into the Baltic and into the White Sea. In the latter, the invading _A. leptodactylus_ is everywhere overcoming and driving out _A. nobilis_ in the struggle for existence, apparently in virtue of its more rapid multiplication.[26]
[26] Kessler (Die Russischen Flusskrebse, l. c. p. 369–70), has an interesting discussion of this question.