GROWTH OF PLANCTONE 175 



by the observations made of late years on the variation 

 from month to month in the volume of the living forms, 

 animals and plants, which constitute the floating or 

 swimming world (planctone) of lakes and rivers. An 

 estimated quantity of water is allowed to pass through a 

 muslin net with meshes fine enough to arrest the living 

 creatures, the volume of the capture is then measured. 

 The diagram on p. 174 illustrates by means of curves 

 the change in volume of the planctone with the season 

 of the year as observed in the case of the Ploen * 

 and the Dobersdorf lakes in Holstein.f The rapid 

 increase of living forms with rise of 

 temperature and the sudden diminution 

 with its fall are sufficiently striking. 



The favourable influence of a less 

 extreme climate is shown as we proceed 

 southwards by the appearance of new 

 elements in the freshwater fauna. The 



._ ,-,.., i FlG - 51. AFresh- 



Melanndae, a group of univalve mol- wa ter Crab, Tkei- 

 luscs, which, during the warmer climate ^^ented'^^n 

 of the Mesozoic period, were inhabi- a Sicilian coin. 

 tants of our own rivers, but have long 

 since deserted them, are represented by 

 numerous species ; crabs belonging to the family Thel- 

 phusidae (sometimes represented on ancient Greek coins) 

 (Fig. 51) occur in the rivers of Southern Europe, and 

 are common in Africa ; and various other Crustacea 

 which we are accustomed to regard as purely marine 

 have acquired a freshwater habitat in the tropics. The 

 same is true of certain bivalve molluscs ; thus Area 

 scaphula lives in the Jumna, near Hamipur, in the 

 interior of India ; a species of piddock, PJiolas rivicola, 

 Sow., is found on floating timber in the River Pantai, 



* See "Zacharias, Biol. Central-blatt," vol. xvi. p. 803, 1896. 

 f- See " Apstein," ibid., vol. xii. p. 485, 1892. 



