CRUSTACEA OF FRESH WATERS 181 



allies in fresh waters elsewhere. In the streams of 

 Southern Brazil and Chili there is found a small 

 Crustacean (Mglea l&vis Plate XXIV.), not unlike 

 the Galatheas of our own coasts, which is interesting 

 as being the only species of the Anomura found in 

 fresh water. Still more remarkable are the Syn- 

 carida, which are represented by two species of 

 "Mountain Shrimps" (see Fig. 84, p. 264) in Tas- 

 mania, and by a third species found near Melbourne. 

 These forms have no near allies among living 

 Crustacea, but appear to be related, as will be shown 

 in a later chapter, to certain fossil Crustacea found 

 in Palaeozoic rocks. 



Belonging to a different category from any of 

 those mentioned are certain Crustacea closely allied 

 to, or identical with, species living in the sea, which 

 inhabit inland lakes where no direct passage from 

 the sea is now possible. Attention was first called 

 to these in the case of some of the large lakes of 

 Sweden, in which Professor Loven found some Crus- 

 tacea My sis relicta (see Fig. 16, p. 47), Mesidotea 

 entornon, Pontoporeia affinis almost or quite identical 

 with species inhabiting the Baltic, the Arctic Ocean, 

 and the North Atlantic. There is geological evidence 

 to show that these lakes were once fjords, or arms 

 of the sea, and have become cut off from com- 

 munication with the Baltic by gradual elevation of 

 the land. The marine animals which they contained 

 would thus be imprisoned, and as the water became 



