THE ANIMALS OF LAKES AND EIVERS 185 



higher Crustacea largely so. The great group of the 

 Coelentera is very scantily represented in fresh water, 

 where also there are but few sponges. Fresh-water fish 

 also are few in number as compared with their marine 

 allies, though they include some peculiar forms, like 

 the ganoids (e.g. sturgeon, bony pike, &c.), and the 

 dipnoi or double-breathers. It is, however, interesting 

 to note that of these pecuhar fish all the three Hving 

 dipnoi, and at least one ganoid (Polypterus) have 

 accessory breathing organs which enable them to breathe 

 air, as well as gUls. This fact, taken in conjunction 

 with the relatively large numbers of air-breathing 

 invertebrates, suggests that one diffictdty in colonizing 

 fresh water has always been its relative deficiency in 

 oxygen. In the general case this apphes, of course, to 

 lakes ahd inland seas rather than to rivers, whose 

 water is oxygenated by its motion. But where a 

 marked dry season occurs, as in many parts of the 

 globe, the rivers may periodically cease to flow, and 

 be represented by a series of stagnant pools, whose 

 waters become very fetid— conditions highly unfavour- 

 able to animal Mfe. 



In the lakes of temperate regions, where the surface- 

 water periodically cools below the point of maximum 

 density of water, a vertical circulation is produced which 

 carries down oxygen to the deeper layers of the water, 

 at least at certain seasons. This makes life in the 

 deeper layers possible, though some American experi- 

 ments suggest that the oxygenation of these deeper 

 layers occurs only in spring and autumn, as the ice 

 melts in the former case, and before it is formed in 

 the latter. In the summer the heating of the surface- 

 water gives rise to a stratification into two layers, 

 when the warm oxygenated surface-water Ues upon a 



