vii LITTORAL AND PELAGIC REGIONS OF LAKES 2O/ 



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 nut 

 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. 

 0. S. Gibson and myself found that the weedy littoral region of 

 Grasmere contained almost entirely large quantities of the 

 ( 'ladoceran Eurycercus lamcllatus, and a number of f//r/<y/,s 

 //'.sews 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 1 region is distinguished from the littoral by the 

 gn-atcr purity and transparency of its waters, and by I he 

 relative stability of the temperature, the annual range, even a,i, 

 the surface, in ( leneva being from 4 20 C., while at 100 metres 

 the water has a uniform temperature of 4 or 5 < '. The upper 

 strata are, of course, brightly illuminated, but at L'O 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 lite 

 swimming freely in the water, sometimes at the surface and 

 sometimes iii the deeper strata. They consist chietly of 

 Diatoms, I'roto/oa, Uotifera, and ('ruslacea. The pelagic 

 Crustacea, especially the Cladocera. are ot'leii the mosl curiously 

 and delicately built creatures. Lr/i/nilin'i/ hyalina, which is 

 quite transparent, is the largest of them, attaining l" three- 

 quarters of an inch in length, thoii-h Bythotrephes /<i/i>//- 

 manus is nearly as large if we include the immense spine 

 which terminal''* the body. //>/,,//, il'i u m gibberum, \\hich is 



1 Consult A]iMrin, '"I>ns Siiss\vassrr|,];uiklun," Ki.'l anl I,.-i]>/i.v, 1V"'> . and 

 . IlydroMoloyic n. riiiLioiiLnn<J<. iiumrmus |>.i|n i . 



