FRESH WATER ANIMALS 21 5 



water fishes, perhaps some gastropods, and a few of the me- 

 dusae might be called pelagic. But poor as it is the composi- 

 tion of this fauna is complex, and like the corresponding 

 fauna in the sea it changes in its aspect with locality and 

 depth and is subject to great seasonal and other variations. 



Every body of fresh water has its own peculiar features, yet 

 as a whole the fresh water fauna is singularly constant every- 

 where. 



Let us briefly recapitulate the animals of fresh water. Most 

 characteristic are the insects, the phyllopod crustaceans, the 

 gastrotrichas, the frogs and toads and salamanders, and the 

 air breathing fishes, which are not found in the sea; the rotifers, 

 leeches and aquatic earth-worms, too, are characteristic of 

 fresh water, though there are a few sea types of all. There are 

 very few kinds of jelly-fishes, hydroids, polyzoans, nemerteans 

 or sponges in fresh water, though the few species are usually 

 abundant where they live. Our fresh water jelly-fish is probably 

 much commoner than is supposed; those which I have seen 

 kept wholly to the bottom and only swam when stirred up 

 with a stick. There are no molluscs in fresh water except for 

 snails and river clams and in a few places ship-worms, or tere- 

 dos, almost no annelids except for those of the earth-worm type, 

 and no barnacles or allied crustaceans. Nematodes are numer- 

 ous, and there are many flat-worms and very many protozoans. 

 There are very many kinds of fishes, numerous reptiles, and a 

 few seals, sea-cows and cetaceans. 



Perhaps it should be pointed out that many nematodes and 

 crustaceans, some frogs, nemerteans, flat-worms, snails, and, 

 rarely, bivalves, Hve on land or in moist earth, while the cysts 

 or eggs of polyzoans and of rotifers are dusted everywhere. 



It is curious that of the two crustacean groups most interest- 

 ing from the palaeontological point of view one, including the 

 fairy shrimps and their allies, occurs only in fresh, or at least 

 non-marine, waters, while the other, including the king or 

 horse-shoe crabs, is found only in shallow water in the sea; 

 and further that the fishes similarly most interesting, the lung 



