2l6 CRUSTACEA 



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 aljsent 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 Eiver-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, Dia'ptomus 

 being entirely absent. Of the Cladocera there are a number of 

 pelagic genera (e.g. Le'ptodora, Holopcdium, Bythotrephes) entirely 

 confined to the lakes of the northern hemisphere. The distribu- 

 tion of Bosmina is interesting. This genus is distributed all 

 over the nortli 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 iiistance 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 Neonipliargus, intermediate in its characters between the 



' Daday, Term^ Filzetek, xxv., 1902, pp. 101 and 436. 

 - Daday, Blbliotheca ZooJogim, Heft 44, 190;"). 



