THE FRESU-WATER CLAM 225 



that permanently reside in and are best fitted for the river 

 habitat have either clinging organs or a high specific gravity 

 and live on the bottom, or else they burrow into the bottom 

 mud. Consequently, we find in rivers many attached animals 

 (Fig. 216). Thus there are sponges, the httle hydra which is 

 referred to in Chapter XIX, Bryozoa, and, in certain parts of 

 the world, even barnacles. Among the animals that can cling 

 by means of an adhesive surface are a great variety of snails, 

 whose powerful foot enables its possessor to adhere to the rocky 

 bed of rivers. Here too one finds flatworms and leeches and 

 the larvse of certain insects which live in nets cemented to the 

 stones. Among animals that are provided with other means 

 of prehension are crayfish, which live under stones and in pools. 

 In the mud or sand at the bottom are found numerous worms 

 and, especially in limestone countries, fresh-water clams. 



All these different kinds of fluviatile animals have been 

 derived directly or indirectly from the ocean and have found 

 themselves well adapted to live in a rushing current. It will 

 be worth while to see how they have reached their present 

 home. 



The fresh-water sponges belong to one small group, whereas 

 their marine allies are vastly more varied. The origin of fresh- 

 water sponges is, however, obscure. Of the Hydra-like animals 

 there are a few that are found chiefly in estuaries ; and in such 

 locations they show us that representatives of the group are 

 still emigrating from the sea. So, too, in the case of the Bryozoa 

 there are several species alhed to marine families of that class 

 that are found in the lower courses of streams and are evidently 

 just beginning to become emancipated from the sea. The 

 river mollusks belong to several different groups which have 

 migrated independently into the streams. Thus in the South- 

 Q 



