104 THE WONDER OF LIFE 



plumicornis, and also the larval stages of some shore forms, 

 e.g. of the bivalve Dreissensia, In the transparency, the 

 delicacy of build, and the occasional presence of long pro- 

 cesses — ^beUeved to be useful in drifting — we see adaptations 

 to the open water life. 



The success of a lake depends to a large extent upon 

 the open water population, and waxes and wanes with its 

 vicissitudes. A few forms are almost uniformly abundant 

 all the year round, but the majority show a marked periodi- 

 city. Thus the Eotifer Synchceta has its chmax in spring, 

 and there may be about three miUions to the square yard 

 in April. The well-known Diatom, Melosira varians, has 

 two maxima in the year, one in July and one in October, 

 and may attain in the last-named month to the astonishing 

 abimdance of about 7,000 milUons to the square yard. 

 The slimy Alga, Clathrocystis aeruginosa, has its climax 

 about August, with about 500 miUions to the square yard. 

 Others, again, have their maximum in winter, such as the 

 Copepod Crustacean, Diaptomics gracilis, whose propor- 

 tionate representation for the four seasons is indicated by 

 the figures— 760 for April, 7,900 for August, 31,160 for 

 September, and 121,290 for January. The broad fact to 

 be realized is that the upper layers of the open water are 

 the chief productive areas, where the Algae utihze the 

 energy of the sunUght to build up the carbon-compounds 

 which form the fundamental food supply of all the lacustrine 

 population. 



The third great region is that of the greater depths of 

 the lake, a region of imiformity, where there is neither 

 day nor night, where the temperature is low and relatively 

 uniform, where the pressure is very great, where there are 

 no movements apart from hfe, and where there is usually 



