VII LITTORAL AND PELAGIC REGIONS OF LAKES ZOF, 
agitation of its waters, the affluence of streams and drainage, 
and the constant changes of temperature. The water in this 
region generally contains a good deal of solid matter in sus- 
pension, while the shelving banks of the lake support a wealth 
of vegetable growth, both of Algae and of Phanerogams, down 
to about 20-25 metres. At this depth the daylight does not 
penetrate sufficiently to admit of the growth of green plants, so 
that this region marks the limit, both physical and biological, 
between the littoral and the abyssal zones. In this littoral 
region there flourish a great quantity of Entomostraca, most of 
which are also found in small ponds where similar conditions 
of life prevail—the pelagic species only penetrating rarely, and 
by accident, into its waters. At the beginning of July Mr. H. 
O. 8. Gibson and myself found that the weedy littoral region of 
Grasmere contained almost entirely large quantities of the 
Cladoceran Hurycercus lamellatus, and a number of Cyclops 
fuscus and C. strenuus. In the littoral zone of large lakes, 
Amphipods, Isopods, and fresh-water shrimps may also be met 
with, but this applies more to the lakes of the Tropics and of 
the Southern Hemisphere. 
The pelagic’ region is distinguished from the littoral hy the 
ereater purity and trausparency of its waters, and by the 
relative stability of the temperature, the annual range, even at 
the surface, in Geneva being from 4°_20° C., while at 100 metres 
the water has a uniform temperature of 4° or 5°C. The upper 
strata are, of course, brightly illuminated, but at 20 metres 
there is hardly sufficient light for green plants to grow, and at 
100 metres it is completely dark. The inhabitants of this 
region, known collectively as plankton, spend their whole life 
swimming freely in the water, sometimes at the surface and 
sometimes in the deeper strata. They consist chiefly of 
Diatoms, Protozoa, Rotifera, and Crustacea. The pelagic 
Crustacea, especially the Cladocera, are often the most curiously 
and delicately built creatures.  Leptodora hyalina, which is 
quite transparent, is the largest of them, attaining to three- 
quarters of an inch in length, though Sythotrephes longi- 
manus is nearly as large if we include the immense spine 
which terminates the body.  Holopedium gibberum, which is 
1 Consult Apstein, ‘‘Das Siisswasserplankton,” Kiel and Leipzig, 1896 ; and 
Arch. f. Hydrobiologie u. Planktonkunde, numerous papers. 
