Fy ih Al CRUSTACEA CHAP. 
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 com- 
plete 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 immi- 
grants 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, MZysis 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 7m situ, and their origin from marine ancestors is 
lost in an obscure past. The Crustacean fauna of the Caspian 
