2 14 CRUSTACEA 



has ParanephroiJS, 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 Asktcojms 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 Eiver-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, 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 



