CRUSTACEA OF FRESH WATERS 165 



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. A rtemia 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 ot haemoglobin, the red colouring matter of 

 the blood of Vertebrates, which is important in the 

 process of respiration. 



The smaller Branchiopoda known as " Water-fleas," 

 forming the order Cladocera, are abundant every- 

 where in fresh water. Daphnia pulex and other 

 speces of the genus, and the little Lynceidae, of 

 which Chydorus sphcericus (Fig. 56) is the commonest 

 species, are to be found in ponds and ditches, and 



