140 A REVISION OF THE ASTACIDA. 
A. torrentium, A. leptodactylus, and A. Colchicus. The first of these possesses 
two rudimentary pleurobranchiw on each side of the body, like A. padlipes ; 
of the anterior pleurobranchia I find not the slightest trace. In A, leptodac- 
fylus and A, Colchicus three are present, although in the former the second 
and third are very short. 
In so far as the reduction of the branchiz can be taken as a clew to the 
affinities of the various species of European Astaci, A. padipes and A. torren- 
tium are the furthest removed from the primitive form. This accords with 
Huxley’s suggestion, that the Ponto-Caspian species are the modern repre- 
sentatives of the original Eastern stock, while the “Stone Crayfishes ” * 
represent an ancient offshoot, or western wave of migration, which was 
followed by an invasion of A. fluviatilis, just as at the present day, according 
to Kessler, the latter species is in its turn succumbing to A. leptodactylus in 
the Baltic area.t Following up this line of thought, we must conclude that, 
of the two species A. ¢orrentium and A. pallipes, the former represents the 
older offshoot from the original stock, an offshoot which has retreated before 
the invading A. pallipes to the mountain regions of Central Europe. For not 
only in the condition of its branchiz, but also in its general form, A. pallipes 
stands between A. forrentium and A. fluviatilis. $ 
In the European Astaci the first abdominal segment of the female carries 
a pair of small, simple appendages. They are smaller than in the genus 
Cambarus, but not aborted, § as in the American Astaci, in Cambaroides, and 
in the Parastacinze of the Southern hemisphere. 
In A. forrentium and A. pallipes the first abdominal appendages of the 
male are divided into two lobes at the tip by a shallow cleft, the inner and 
the outer parts being of about equal length. In the other species the outer 
part is truncate, while the inner part projects beyond it. In A. pachypus and 
A. Colchicus the projecting tip of the inner part is longer than in A. fluviatilis 
and A. leptodactylus. 
The European Astaci, including A. Colchicus from the Rion River, Trans- 
caucasia, may be distinguished by the following table : — 
* Te. A pallipes and A. torrentium, which Huxley confounds together as one species. 
+ Vide Huxley, op. cit., p. 821. 
{ Klunzinger (op. cif., p. 831) goes so far as to assert that 4. padlipes is more closely related to 4. flu- 
viatilis than to A. torrentium, but it does not seem so to me. A. fluviatilis, A. leptodactylus (including the 
form 4. angulosus), A. pachypus, and A. Colehicus form a natural group of closely related species opposed 
to the group containing the two Western species, 4. pallipes and A. terrentium. 
§ Huxley affirms, however, that these appendages are sometimes wanting in English crayfishes (4. pad- 
lipes). The Crayfish, p. 146. 
