DISTRIBUTION 141 



fully penetrated into fresh water and so may be dealt with more 

 systematically. 



The Notostraca, Anostraca and Conchostraca, sometimes known 

 collectively as the phyllopods, have a world-wide distribution, 

 though within each group there is often a division into distinct 

 geographical units. Longhurst's study of the Notostraca has shown 

 that in the genus Triops (fig. 5) the four species occupy different 

 parts of the globe, with only two of them overlapping. Triops 

 granarius is the most widespread member of the genus, being found 

 from South Africa across Asia to China; Triops cancriformis is 

 found from North Africa and Western Europe to Russia and India; 

 T. longicaudatus is an American species extending across the 

 Pacific to Hawaii, Japan and New Caledonia, and T. australiensis is 

 found in Australia and Madagascar. These species seem to have 

 divided the world fairly evenly between them. The peculiar dis- 

 tribution of T. australiensis is a feature which will be echoed when 

 the distribution of fresh-water crayfish is described. 



It is characteristic of most phyllopods that they are inhabitants 

 of temporary pools, usually in arid climates. They cannot with- 

 stand competition from the insects and more highly evolved 

 Crustacea which are present in permanent pools. The few species 

 of phyllopods which are found in permanent pools usually appear 

 for a short time in the spring, grow very rapidly, and produce 

 resting eggs before the other inhabitants of the pools have become 

 properly active after their winter inactivity, 



The Cladocera have but a few genera, such as Podon, Penilia and 

 Evadne, in the sea, but in fresh water they are among the most 

 abundant and prolific of all Crustacea. Various species occupy 

 different parts of the environment. Some, such as Holopediam and 

 Leptodora, are purely planktonic, others associate with weeds, yet 

 others such as Macrothrix and Camptocercus skim over the bottoms 

 of pools, and some, such as Ilyocryptus, burrow in mud and have 

 lost the power to swim. The systematics of the group are difficult, 

 because the form of the body often changes with the seasons, and 

 from place to place. Because of these systematic difficulties it is hard 

 to get a clear geographical picture of the distribution of Cladocera. 

 All that can be safely said is that there appears to be a cosmopolitan 

 group of species, with Chydorus sphaericus as their most ubiquitous 

 member, a pan-tropical group, including such species as Macrothrix 

 triserialis, and Ilyocryptus spinifer, and apart from this there are 

 various species which are so far known to have limited distributions. 



