Two groups of Brachyura, viz. the Thelphusidae and the Sesarminae (a sub-family of the Grapsidae), are fresh-water. Thelphusa fluviatilis is an inhabitant of North Africa, and penetrates into the temperate regions of the Mediterranean, and is said to be exceedingly common in the Alban Lake near Rome. Both these families have representatives on land, e.g. Potamocarcinus in Central and South America, and certain species of Sesarma, and the closely related Gecarcinidae of the West Indies.
The remaining families to be dealt with are the two Crayfish families—the Astacidae and the Parastacidae—which live in rapidly moving rivers and streams, and occasionally in lakes. A few species of both families have taken to a subterranean mode of life, and excavate burrows in the earth, e.g. the Tasmanian Crayfish, Engaeus fossor. The distribution of the Crayfishes has long engaged the attention of naturalists. It was first seriously studied by Huxley,[[174]] and has subsequently been followed up, especially in North America, by Faxon[[175]] and Ortmann,[[176]] but our knowledge of the South American and Australian forms is still very incomplete. The Astacidae inhabit the northern temperate hemisphere, the Parastacidae the southern temperate hemisphere, the tropical belt being practically destitute of Crayfishes. Of the Astacidae the genus Astacus (Potamobius), including the common Crayfishes of Europe, occurs in Europe and in North America west of the Rockies. The genus Cambaroides, which in certain respects approaches Cambarus, is found in Japan and Eastern Asia. The very large genus Cambarus, on the other hand, only occurs in North America east of the Rockies, so that Cambaroides occupies a very isolated position. The occurrence of a Cambarus, C. stygius, in the caves of Carniola, has been recorded by Joseph, so that it would appear that this genus had a much wider range formerly than now.
Of the southern temperate Parastacidae, Australia and Tasmania have the genera Astacopsis and Engaeus; New Zealand has Paranephrops, South America Parastacus, and Madagascar Astacoides. The last named genus is rather isolated in its characters, possessing a truncated rostrum and a highly modified branchial system, but it agrees with all the other Parastacine genera, and differs from the Astacidae in the absence of sexual appendages on the first abdominal segment, and in the absence of a distinct lamina on the podobranchiae. The largest crayfish in the world is Astacopsis franklinii, found in quite small streams on the north and west coast of Tasmania. Specimens have been caught weighing eight or nine pounds, and rivalling the European Lobster in size. Crayfishes appear to be entirely absent from Africa.
It seems reasonable to suppose that the two families of Crayfishes characteristic respectively of the northern and southern hemispheres have been independently derived from marine ancestors, which have subsequently become extinct. Their complete absence in the tropics is striking, and Huxley drew attention to the fact that it is exactly in those regions where the Crayfishes are absent that the other large fresh-water Malacostraca are particularly well developed, and vice versa. Thus the large fresh-water Prawns are typically circumtropical in distribution, while the South African rivers abound with River-crabs, which, in general, are found wherever Crayfishes do not occur.
A few of the more interesting features in connection with the distribution of fresh-water Crustacea have now been touched upon. With regard to the origin of this fauna, we can see that a number of the species are comparatively recent immigrants from the sea, working their way up the estuaries of rivers, a proceeding which can be observed to be taking place to-day in a district like the Broads of Norfolk. Others, again, but these are few, appear to be true relict marine animals left stranded in arms of the sea that have been cut off from the main ocean, and have been gradually converted into fresh-water lakes and seas. Such are, perhaps, Mysis relicta and the rich Gammarid fauna of Lake Baikal, a lake that, in the presence of Seals, Sponges, and other marine forms, has clearly retained some of the characters of the ocean from which it was derived. The majority of the fresh-water species, however, have probably been evolved in situ, and their origin from marine ancestors is lost in an obscure past. The Crustacean fauna of the Caspian Sea[[177]] shows us in an interesting manner the effects of isolation and changes in salinity, etc., on the inhabitants of a basin which once formed part of the ocean. The waters of the Caspian Sea are not fresh, but they are on the average about one-third as salt as that of the open ocean. The Crustacea, described by Sars, belong to undoubtedly marine groups, e.g. the Mysidae, Cumacea, and Amphipoda Crevettina, but the remarkable feature of these Caspian Crustacea is the great variety of peculiar species representing marine genera which are very poorly represented in the sea, thus indicating that the variety of the fauna is not due to a great variety of species having been shut up in the Caspian Sea to begin with, but rather that, after the separation from the sea, the isolated species began to vary and branch out in the most luxuriant way—whether from lack of competition or owing to the changing conditions of salinity it is difficult to say. As an example, the Cumacea of the Caspian Sea are ten in number, all belonging to peculiar genera related to Pseudocuma, except one species which is included in that genus. These Caspian forms make up the Family Pseudocumidae, which contains in addition only two marine forms of the genus Pseudocuma (see p. [121]). A very similar condition is found in the numerous Amphipods of the Caspian Sea. Considering the enormous changes that must have taken place in the distribution of land and water even during Tertiary times, it is astonishing that the fresh waters of the world do not contain more species in common with the ocean, but it must be considered that the limited area and comparatively uniform conditions of fresh-water lakes and streams would only permit a limited number of these forms to survive which could most easily adapt themselves to the changed conditions. And these would in all probability be the littoral species that were in the habit of passing up into the brackish waters of estuaries and lagoons, so that the uniform and limited nature of the fresh-water fauna can be accounted for to a certain extent by this hypothesis.
We have seen in dealing with the marine Crustacea of the littoral zone that the chief condition determining their distribution is temperature, and that the world may be divided into three chief areas of distribution for these animals, viz. the north temperate hemisphere, the tropics, and the south temperate hemisphere. It seems that the same division holds good for fresh-water Crustacea. We have already seen that the Crayfishes follow this rule, being practically absent from the tropics, and represented in the two temperate hemispheres by two distinct families, the Astacidae in the north and the Parastacidae in the south. Characteristic of the tropical belt are the absence of Crayfishes and the great development of Prawns and River-crabs. In the case of Entomostraca the great majority of the genera are cosmopolitan, especially those which live in small bodies of water liable to dry up, because these forms always have special means of dissemination in the shape of resting eggs which can be transported in a dry state by water-birds and other agencies to great distances; but those genera which inhabit large lakes are more confined in their distribution. The Copepod genus Diaptomus, characteristic of lake-plankton, ranges all over the northern hemisphere and into the tropics, but it is almost entirely replaced in the southern hemisphere by the related but distinct genus Boeckella,[[178]] which occurs in temperate South America, New Zealand, and southern Australia, and was found by the author to be the chief inhabitant in the highland lakes and tarns of Tasmania, Diaptomus being entirely absent. Of the Cladocera there are a number of pelagic genera (e.g. Leptodora, Holopedium, Bythotrephes) entirely confined to the lakes of the northern hemisphere. The distribution of Bosmina is interesting. This genus is distributed all over the north temperate hemisphere in lakes and ponds of considerable size, not liable to desiccation; in the New World it passes right through the tropics into Patagonia,[[179]] the chain of the Andes doubtless permitting its migration. In the tropics of the Old World it is unknown, but it turns up again, as the author recently found, as a common constituent in the plankton of the Tasmanian lakes. There is another instance of a group of Crustacea, characteristic of the north temperate hemisphere, being entirely absent from the tropics, at any rate of the Old World, but reappearing in the temperate regions of Australasia. The commonest fresh-water Amphipods in this region belong to the genus Neoniphargus, intermediate in its characters between the northern Niphargus and Gammarus, but grading almost completely into the latter. Both Niphargus and Gammarus are absolutely unknown from the tropics, but whether, like Bosmina, they occur in the Andes and temperate South America is not known; it seems, however, probable that they have reached Southern Australia by way of South America rather than through the tropics of Asia and Australia, where there is no range of mountains to permit the migration of a group of animals apparently dependent on a temperate climate. The other common fresh-water Amphipod in temperate Australia and New Zealand is Chiltonia, whose nearest ally is Hyalella from Lake Titicaca on the Andes, and temperate South America.
The Anaspidacea and Phreatoicidae, which are so characteristic of temperate Australia, and are generally of an Alpine habit, have never been found in South America, but the Anaspidacea are represented by numerous marine forms in the Permian and Carboniferous strata of the northern hemisphere, so that it is probable that this group reached the southern hemisphere from the north through America.
The distribution of the fresh-water Crustacea, therefore, in the temperate southern hemisphere affords strong evidence in favour of the view that the chief land-masses of this hemisphere, which are at present separated by such vast stretches of deep ocean, were at no very remote epoch connected in such a way as to permit of an intermixture of the temperate fauna of New Zealand, Australia, and South America. While this connexion existed, a certain number of forms characteristic of the northern hemisphere, which had worked through the tropics by means of the Andes, were enabled to reach temperate Australia and New Zealand. The existence of a coast-line connecting the various isolated parts of the southern hemisphere would, of course, also account for the community which exists between their littoral marine fauna. It is impossible to enter here into the nature of this land-connexion which is becoming more and more a necessary hypothesis for the student of geographical distribution, whatever group of animals he may choose, but it may be remarked that the connexion was probably by means of rays of land passing up from an Antarctic continent to join the southernmost projections of Tierra del Fuego, Tasmania, and New Zealand.
TRILOBITA
BY