The genus _Artemia_ (Fig. 55), among the Anostraca, is peculiar in its habitat; for, while most of the Branchiopoda inhabit fresh or brackish water, it flourishes in concentrated brine. In the South of Europe it is found, as it was formerly in England, in the shallow ponds in which sea-water is exposed to evaporation for the manufacture of salt, and in these it occurs in such numbers as to give the water a reddish colour. It is also found in salt lakes, like the Great Salt Lake of Utah, in the United States, and in many other parts of the world. The specimens from different localities often differ considerably, especially in the form of the tail-lobes; but it has been shown that these differences are more or less directly correlated with the degree of salinity of the water in which the animals live, and it is probable that the forms which have been described are all variations of a single cosmopolitan species ranging from Greenland to Australia, and from the West Indies to Central Asia. _Artemia_ is the only one of the Anostraca that is known to be parthenogenetic, some colonies consisting entirely of females, while in others males are abundant. The reddish colour above alluded to is found also in _Branchipus_, _Apus_, and other Branchiopoda, and is due, as Sir Ray Lankester first showed, to the presence in the body-fluids of hæmoglobin, the red colouring matter of the blood of Vertebrates, which is important in the process of respiration. [Illustration: FIG. 56--_Chydorus sphæricus_, A COMMON SPECIES OF WATER-FLEA. × 50. (After Lilljeborg.)]