184 THE ANIMALS OF LAKES AND KIVERS 



the sea as well as in fresh water. Similarly, the bivalve 

 molluscs of the lakes and streams have certainly had 

 marine ancestors. Most of the higher crustaoea live in 

 the sea, and the few crayfish and prawns which live 

 in fresh water are certainly descended from ancestors 

 which lived in the open ocean. 



It is otherwise with most of those numerous forms 

 which, though they live in water, are yet adapted for 

 breathing air. The otter which haunts the salmon 

 rivers has lungs no less than its distant relative the 

 polecat ; the water-tortoises have lungs like the land 

 forms ; the water-beetles and the many insect larvae 

 found in pools, the water-spider, the water-mites, the 

 pond-snails, are all air-breathers, and their peculiar 

 respiratory organs would be inexplicable if we could 

 not assume that their immediate ancestors lived on 

 land. In these cases there is no reason to beheve that 

 the animals have ever sought the sea since the time 

 when their far-off ancestors acquired terrestrial charac- 

 ters. Therefore in possessing some of these forms 

 the fresh-water fauna is not poorer but richer than 

 the sea, which has hardly any insects (cf. p. 176), no 

 air-breathing molluscs, no true spiders, and so on. 

 Again, while most amphibians have a fresh-water 

 larval stage, and some pass much of their life in water, 

 no living amphibian is ever found at any period of its 

 life in the sea. 



But when we come to consider the groups which are 

 richly represented in the sea, the tale is different. The 

 echinoderms are exclusively marine — ^no starfish, sea- 

 urchin, brittle-star, nor sea-hly is known from fresh 

 water. No cuttle hunts its prey in the depths of the 

 great lakes, nor creeps along river bottoms. The poly- 

 chaete worms are almost exclusively marine, and the 



