THE ANIMALS OF LAKES AND RIVERS 199 



which also occurs round the coasts in salt and brackish 

 water, and its presence is doubtless to be explained on 

 the supposition that it is a recent migrant from the 

 sea. But in certain continental lakes, especially those 

 in Denmark and on the North German plain, another 

 variety or species occurs, called Mysis relicta. The 

 modifications which this form displays are believed to 

 indicate that the bodies oi water in which it occurs 

 were once connected with the sea, but have been long 

 separated from it. Such organisms are said to con- 

 stitute a relict fauna, and the lakes in which they occur 

 are called relict lakes (German, Reliktenseen), to indi- 

 cate their supposed former connexion with the sea. 



With the Scottish lochs may be contrasted Lake 

 Balaton or Flatten See, a large shallow body of water 

 in Hungary, connected with the Danube drainage 

 system. The area of this lake is about 250 miles, but 

 its average depth is only 10 feet, and the maximum 

 36 feet. Its fauna has been subjected to detailed 

 investigation. 



As its shallowness makes Lake Balaton a mere pool, 

 we find that here the fauna forms a unity. Necessarily 

 there is no deep-water fauna, and it is not possible to 

 discriminate accurately between httoral and pelagic 

 faunas, as the bottom is everywhere so close to the 

 surface. The total number of species does not differ 

 greatly from that of the Scottish lochs, being about 

 475, including 38 kinds of fish, but exclusive of insects 

 and air-breathing vertebrates. The fish show some, 

 interesting pecuKarities. There are no trout, but the 

 lake contains such common fresh-water forms as perch, 

 gobies, pike, eels, roach, tench, gudgeon, &c., and in 

 addition two forms of special interest. One is the cat- 

 fish caUed ' Wels ' by the Germans {Silurus glanis), 



