PUYTXOPOD CRUSTACEA 53 



to tiiul reported fur au}- i'hyllopod) and Branchipodopsis affiiiis have egg-s requiring freez- 

 ing (though this may not l>e so), and certainly that the eggs of the Bratwhipus and of the 

 Strcptoccpliali require dr\'ing. Tlie finding of Pristiccplialux in the Sargodhar District seems 

 to show that these eggs recjuire drying. It is perfectly possible, of course, that this (and 

 other forms) may have the eggs made ready for hatching by either method indiscriminately. 



Since the eggs will (in most species, perhaps all) withstand long periods of drought, 

 and since ihey are small and light, it might be expected that they would easily and often be 

 transported by the feet of water birds, by the wind, or by other agencies. As a matter of 

 fact, however, the ranges of many species are surprisingly circumscribed, even though ponds 

 and pools offering apparently ideal conditions are to be found a short distance away. Other, 

 perhaps related, species may be very widely distributed, with, so far as has been determined, 

 no more efficient method of dispersal. 



This curious sort of distribution may perhaps test be explained by the presence or 

 absence of various necessary, or destructive, factors in the various environments, the different 

 species differing, of course, in their requirements or sensitivity. Unhappily, very little is 

 known of the particular ecological factors involved, with the exception of Artemia from cer- 

 tain regions, and since the environment of brine-pools and salterns is so special, little light 

 is thrown upon the situation in other genera. Though there is little experimental evidence, 

 it appears probable that temperature is a very important factor, not only in the freezing of 

 eggs, but there is reason to suppose that there are both maximum and minimum, sharply 

 limiting temperatures for many, if not all, species. Thus Heath (1924) has shown that 

 hatching and the early stages of development in Branchinccta occidcntalis may take place at 

 lower temperatures, but that sexual maturity is only attained after the water has risen to a 

 temperature of approximately 22°C., despite abundant food and other suitable conditions. 



The Himalayan Mountain system makes a more or less sharp temperature barrier between 

 North and South, and the higher part of the plateau, even south of the crest is, undoubtedly, 

 for such species as Branchipodopsis affinis essentially similar to Manchuria and Mongolia. 

 Whether this form is to be found in many places between the present known sites, or 

 whether it reached the Himalayan plateau at a period colder than the present, it is impossible 

 to say. The very occurrence of this species in the cold parts of Asia is at present not easy to 

 explain, since all the other 11 known species of the genus are found only in the warm and dry 

 parts of South Africa. The arrival of Branchinccta oricntalis was probably from the North 

 and West, where it now occurs, and in which direction other members of the genus are to be 

 found. Pristicephahis prisctis may be surmised to have come from the North and West also, 

 since its closest relatives are to be found in that direction. Because of its differentiation and 

 restricted range it wnuld appear to have arrived at an earlier date than the other northern 

 forms, however. Branchipus stagnalis, like Strcptoccphalus, i)robably arrived from the West, 

 though it is odd that it has been met with only once, and it appears barely possil^le that it 

 may have been a chance importation through the agency of modern man. 



The localities in which the various forms of Strcptoccphahts are found are shown on the 

 map, Figure 14. The other species are shown in Figure 15. 



