214 



ANIMALS OF LAND AND SEA 



are numerous microscopic plants, the smaller mostly flagellates 

 and algae, corresponding to the microscopic forms in the open 

 ocean. There are many diatoms, as in the sea, and in Lake 

 Mendota in Wisconsin 37,500,000 have been found in a single 

 quart of water. On these, as in the sea, there lives a suspended 

 or pelagic fauna consisting mostly of minute and small crusta- 

 ceans which is more abundant in the colder than in the warmer 

 regions. 



As in the sea, the animals along the shores of ponds and 

 lakes and rivers are the most varied, and with increasing depth 



Figs. 649-658. Jelly-fishes. 

 For explanations of the figures see p. xxxii. 



the number of types rapidly decreases. In the deeper waters 

 more or less characteristic types exist, but there is no such 

 diversified abyssal fauna as there is in the sea. 



The pelagic or permanently floating or suspended fauna of 

 the lakes consists almost entirely of small crustaceans. There 

 are none of the pelagic young of larger bottom-living types, and 

 none of the larger drifting forms such as the tunicates, chaetog- 

 naths, pteropods, heteropods, ctenophores, siphonophores, etc., 

 so characteristic of the pelagic fauna of the sea. Some fresh 



