THE ANIMALS OF LAKES AND RIVERS 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 peculiar fish all the three living 
dipnoi, and at least one ganoid (Polypterus) have 
accessory breathing organswhich enable them to breathe 
air, as well as gills. This fact, taken in conjunction 
with the relatively large numbers of air-breathing 
invertebrates, suggests that one difficulty in colonizing 
fresh water has always been its relative deficiency in 
oxygen. In the general case this applies, of course, to 
lakes and 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 life. 
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 lies upon a 
