VII FRESH-WATER FORMS 20 5 



Bathynomua giganteus and Encephaloides armstrongi, and the 

 loss of the larval development. Owing to the similarity of 

 conditions in the deep sea all over the globe most of its inhabit- 

 ants are universally distributed. It is also a striking fact that 

 species are found in the deep sea of the tropics whose nearest 

 allies occur, not in the littoral seas of the tropics, but in those of 

 the temperate region. This fact has already been alluded to 

 in dealing with the distribution of the Lithodinae. Alcock ^ 

 remarks that between 50-500 fathoms in the Indian Ocean are 

 found Crabs such as Maia, Latreillia, and Homola, regarded as 

 characteristic of the north temperate seas ; the lobster Nephrons 

 andamanica, taken at 150-400 fathoms, is closely allied to the 

 Norwegian N. norioegiea ; and nine species of " Schizopoda," which 

 are certainly temperate forms, occur in the Indian Ocean at 

 depths of 500-1750 fathoms. 



B. Fresh- Water.- 



If we except the Crayfishes and Eiver-crabs, the Crustacean 

 fauna of running water is exceedingly poor, but in all standing 

 fresh-water, from the smallest pond to the large lakes and inland 

 seas, Crustacea, especially Entomostraca, are abundant and charac- 

 teristic, and form an important item in the food of fresh-water 

 fishes. In small ponds a vast assemblage of Cladocera is met with ; 

 these animals multiply with great rapidity by parthenogenesis, 

 especially during spring and summer, but on the advent of 

 untoward conditions sexual individuals are produced, which lay 

 fertilised winter-eggs which lie dormant until favourable condi- 

 tions again arise. As Weismann first pointed out, the frequency 

 with which sexual individuals are produced in the various species 

 is closely correlated with the liability of the water in which 

 they live to dry up ; so that the Cladocera which inhabit small 

 ponds usually have at least two "epidemics" of sexual individuals, 

 one during early summer and the other before the onset of winter. 



Besides Cladocera, the Phyllopoda (e.g. Apvs, Artemia, etc.) 



^ A Naturalist in Indian Seas. 



- Scourfield, J. Quekctt Micr. Club, 1903-4, gives a useful list of British Fresli- 

 Avater Entomostraca. For the identification of fresh-water Cladocera, Lilljeborg's 

 " Cladocera Sueciae," Nov. Act. Reg. S'oc. Upsalensis, 1901; for Copepoda, Schmeil's 

 " Siisswasser Copepoden," in Bibliotheca Zoologica, iv., v., and viii., 1892, 1893, 

 and 1895 are recommended. 



