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 — ^believed to be useful in drifting — we see adaptations 

 to the open water hfe. 



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 Rotifer Syrwhceta has its chmax in spring, 

 and there may be about three millions 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 astonishiag 

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

 The slimy Alga, Clathrocystis aeruginosa, has its chmax 

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

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

 Copepod Crustacean, Diaptomus 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 Algse utiHze the 

 energy of the sunlight to build up the carbon-compounds 

 which form the fundamental food supply of all the lacustriae 

 population. 



The third great region is that of the greater depths of 

 the lake, a region of uniformity, 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 life, and where there is usually 



