216 CRUSTACEA CHAP. 
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 Ento- 
mostraca 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 dissemina- 
tion 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,' which 
occurs in temperate South America, New Zealand, and southern 
Australia, and was found by the author to be the chief in- 
habitant 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 distribu- 
tion of Bosmina is interesting. This genus is distributed all 
over the north temperate hemisphere in lakes and ponds of con- 
siderable size, not liable to desiccation; in the New World it 
passes right through the tropics into Patagonia,’ 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 
1 Daday, Termés Fiizetek, xxv., 1902, pp. 101 and 436. 
2 Daday, Bibliotheca Zoologica, Heft 44, 1905. 
