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 of 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 Platten 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 littoral 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 peculiarities. 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 called ‘Wels’ by the Germans (Silurus glanis), 
