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.