Ill] 



OF THE BRINE-SHRIMPS 



253 



curve (Fig. 78). In short, the density of the water is so clearly 

 "specific," that we might briefly define Artemia jelskii, for instance, 

 as the Artemia of density 1000-1010 (NaCl), or all but fresh water, 

 and the typical A. salina (or principalis) as the Artemia of density 

 1018-1025, and so on*. 



Koppeniana 

 160 """^ 



140 



120 



100 



1000 



1020 



1080 



1100 



1040 1060 



Density of water 

 Fig. 78, Percentage ratio of length of abdomen to cephalothorax 

 in brine -shrimps, at various salinities. After Abonyi. 



These Artemiae are capable of living in waters not only of great 

 density, but of very varied chemical composition, and it is hard to 

 say how far they are safeguarded by semi-permeabihty or by specific 

 properties and reactions of the living colloids "j". The natron-lakes, 



* Different authorities have recognised from one to twenty species of Artemia. 

 Daday de Dees {Ann. sci. nat. 1910) reduces the salt-water forms to one species 

 with four varieties, but keeps A. jelskii in a separate sub-genus. Kuenen suggests 

 two species, A. salina and gracilis, one for the European and one for the American 

 forms. According to Schmankewitsch every systematic character can be shewn 

 to vary with the external medium. Cf. Professor Labbe on change of characters, 

 specific and even generic, of Copepods according to the ^H of saline waters at 

 Le Croisic, Nature, March 10, 1928. 



t We may compare Wo. Ostwald's old experiments on Daphnia, which died in 

 a pure solution of NaCl isotonic with normal sea-water. Their death was not to 

 be explained on osmotic grounds; but was seemingly due to the fact that the 

 organic gels do not retain their normal water- content save in the presence of such 

 concentrations of MgClj (and other salts) as are present in sea-water. 



