Pelagic Fauna of Freshwater Lakes. 321 
OsrRAcopDA : Cypris ovum. 
CLADOCERA: Sida crystallina; Daphnella brachyura; Daphnia 
pulex, D. magna, D. longispina, D. hyalina, D. cris- 
tata, D. galeata, D. quadrangula, D. mucronata; Bos- 
mina longirostris, B. longispina, B. longicornis ; Bytho- 
trephes longimanus ; Leptodora hyalina. 
CorpePopA: Cyclops coronatus, C. quadricornis, C. serrula- 
tus, C. tenuicornis, C. brevicornis, C. minutus ; Hetero- 
cope robusta; Diaptomus castor, D. gracilis. 
If we were to cite all the animals which are found in the 
pelagic region of the lakes we should have to add the insec- 
tivorous fishes which feed upon these little Entomostraca, 
especially the Coregoni, and, further, the predaceous fishes 
(trout, pike, &c.) which attack the Coregont; we should also 
have to add the Infusoria (Vortzcella convallaria) which live 
upon the pelagic Algz; and, finally, we should have to cite 
. the animals which dwell far from the shores or rise from the bot- 
tom, and are thusfound occasionally in the pelagicregion, such 
as Atax crassipes (Pavesi, Asper), larvee of Diptera, and Pisc?- 
cola geometra (Forel). All these animals, however, appear only 
occasionally and as accessories in the pelagic fauna, which in 
reality embraces only the above enumerated Entomostraca * ; 
these alone show the peculiar characters of pelagic animals. 
In its general features the pelagic fauna is similar in all the 
countries and lakes of Kurope that have as yet been investi- 
gated, from the lakes of the plains to those of the Alps, and 
from the Scandinavian countries to Southern Italy and the 
Caucasus. Itis, however, seldom represented in any one lake 
by all the animals of the fauna. Thus, for example, the 
pelagic fauna of the Lake of Geneva in the years 1874-78, 
during which I investigated it, only possessed the following 
species :—Diaptomus castor, Daphnia hyalina, D. mucronata, 
Bosmina longispina, Sida crystallina, Bythotrephes longi- 
manus, and Leptodora hyalina. From this poimt of view 
Pavesi has very carefully examined the Italian lakes, and 
given for each of them a table of the species captured. In 
estimating these tables, however, Weismann’s observations 
must be taken into account. This naturalist has shown that 
the different species of Cladocera present a yearly periodicity ; 
that during many seasons they disappear more or less com- 
* Perhaps Atax crassipes, which Pavesi sometimes observed in the 
pelagic region of the Italian lakes, and which Asper detected in the Lake 
of Zurich, must be regarded as a species belonging to the pelagic fauna. 
It is a swimming water-mite, and, according to these observers, the 
specimens captured were almost transparent. 
