Another circumstance is still more remarkable. The West American crayfishes are but little more different from the Pontocaspian crayfishes, than these are from Astacus torrentium. On the face of the matter, one might therefore expect the Amurland and Japanese crayfishes, which are intermediate in geographical position, to be also intermediate, morphologically, between the Pontocaspian and the West American forms. But this is not the case. The branchial system of the Amurland Astaci appears to be the same as that of the rest of the genus; but, in the males, the third joint (ischiopodite) of the second and third pair of ambulatory limbs is provided with a conical, recurved, hook-like process; while, in the females, the hinder edge of the penultimate thoracic {312} sternum is elevated into a transverse prominence, on the posterior face of which there is a pit or depression.[34]
In both these characters, but more especially in the former, the Amurland and Japanese Astaci depart from both the Pontocaspian and the West American Astaci, and approach the Cambari of Eastern North America.
FIG. 78.—Cambarus (Guatemala) penultimate leg. cxp, coxopodite; cxs, coxopoditic setæ; pdb, podobranchia; bp, basipodite; ip, ischiopodite; mp, meropodite; cp, carpopodite; pp, propodite; dp, dactylopodite.
In these crayfishes, in fact, one or both of the same pairs of legs in the male are provided with similar hook-like processes; while, in the females, the modification of the penultimate thoracic sternum is carried still further and gives rise to the curious structure described by Dr. Hagen as the “annulus ventralis.”
[34] Kessler, l. c.
In all the Cambari, the pleurobranchiæ appear to be entirely suppressed, and the hindermost podobranchia has no lamina; while the areola is usually extremely narrow. The proportional size of the areola in the Amurland {313} crayfishes is not recorded; in the Japanese crayfish, judging by the figure given by De Haan, it is about the same as in the western Astaci. On the other hand, in the West American crayfishes it is distinctly smaller; so that, in this respect, they perhaps more nearly approach the Cambari. Unfortunately, nothing is known as to the branchiæ of the Amurland crayfishes. According to De Haan, those of the Japanese species resemble those of the western Astaci: as those of the West American Astaci certainly do.
With respect to the Parastacidcæ; in the remarkable length and flatness of the epistoma, the crayfishes of Australia, Madagascar, and South America, resemble one another. But in its peculiar truncated rostrum (see fig. [65]) and in the extreme modification of its branchial system, which I have described elsewhere, the Madagascar genus stands alone.
The Paranephrops of New Zealand and the Fijis, with its wide and short epistoma, long rostrum, and large antennary squames, is much more unlike the Australian forms than might be expected from its geographical position. On the other hand, considering their wide separation by sea, the amount of resemblance between the New Zealand and the Fiji species is very remarkable.