320 Prof. F. A. Forel on the 
XXXIU.— The Pelagic Fauna of Freshwater Lakes. 
By Prof. F. A. ForEL*. 
In the years 1860-1870 the Scandinavian naturalists dis- 
covered a peculiar fauna, consisting essentially of swimming 
Entomostraca, which inhabits the pelagic region of lakes. I 
will endeavour to give a brief summary of this chapter of 
general zoology, which has been of late years repeatedly in- 
vestigated, and has led to some new and interesting results f. 
This fauna is not very numerous in species; but the num- 
ber of individuals of the different speciesis enormous. I give 
here an enumeration of the species that have been found :— 
* Trans'‘ated from the ‘ Biologisches Centralblatt, Band i. p. 299. 
+ Literature :-— 
W. Lilljeborg (“ Beskrivning &c.,” Gifversigt afK.Vetensk. Akad. Forh. 
1860) described the genera Bythotrephes and Leptodora, which are pe- 
culiar to this fauna.. (Bythotrephes was first discovered by Leydig in 
1857, in the stomachs of Coregoni from the Lake of Constance; he 
erroneously placed its habitat in the depths of the lake.) 
From 1861-1865 G.O. Sars described numerous pelagic Entomostraca in 
the Norwegian lakes. (‘‘Om Crustacea Cladocera,’ Forh. i Vidensk. Selsk. 
Christiania, 1861 ; ‘Omen i Sommeren 1862 foretagen zoologisk Reise,’ 
Christiania, 1863; ‘Norges Fervands Krebsdyr,’ Christiania, 1865.) 
In 1866 Scheedler (“ Cladoceren des Frischen Haffs,’ Wiegmann’s 
Archiv, 1866) described Daphne which he had captured in the Frischen 
Haff. 
In 1867 P. E. Miller (¢ Danmark’s Cladocera,’ 1867; “ Cladocéres des 
grands lacs Suisses,” Arch. des Sci. Phys. et Nat. Genéve, 1870) ascer- 
tained the presence of this fauna in the Danish lakes; in 1868 he also 
found it in the Swiss lakes. 
In 1871 Fri¢ investigated the distribution of these Entomostraca in the 
Bohemian lakes (“‘ Fauna der Bohmerwaldseen,” Gesellsch. der Wissensch. 
Prague, 1871). 
From 1874 to 1879 I myself investigated them in the Swisslakes (“ Maté- 
riaux pour la faune profonde du lac Léman—Faune pélagique XXXII., 
Flore pélagique XX XITI., Transparence del’eau VII. et XXX VIII.,” Bull. 
Soc. Vaud. Sci. Nat. tomes xiii. & xiv. Lausanne, 1876; ‘ Variations de 
la transparence de l’eau,” Arch. des Sci. Phys. et Nat. lix., Genéve, 1877). 
From 1874 to 1879 A. Weismann published his fine memoirs upon 
the natural history of the Daphnidé (“ Beitr. zur Naturg. der Daphniden,” 
Zeitschr. fiir wiss. Zool., 1874-79), founded upon hisinyestigations in the 
Lake of Constance. In 1877 he gave, in a popular discourse, “ Das 
Thierleben im Bodensee ” (Lindau, 1877), an excellent general descrip- 
tion of the different faunas that inhabit the lake, and especially of the 
pelagic fauna. 
In 1877 Pavesi discovered the marine fauna in the Italian lakes (Bull. 
entom. 1879 ; Rendiconti R. Ist. Lomb. ser. 2, xii. f. 11, 12, 16). 
In 1879 Brandt collected these forms in the Gotschai lake, in the Cau- 
casus (Bull. Acad. St. Pétersh. 1880). 
S. T. Smith has detected them in Lake Superior, in North America. 
G. Asper (‘ Gesellsch. kleiner Thiere der Schweizer. Seen,’ Zurich, 1880) 
studied the pelagic and abyssal fauna of the different Swiss lakes. 
