148 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



by a rapid stream of pure drinkable fresh-water, and opened their 

 shells to it. Many marine Bryozoa occur also in fresh water. 

 Among the Annelida the case seems to be rarer, and I have 

 only been able to find one instance mentioned in books by 

 Leidy, who discovered a worm, Manayunkia, belonging to the 

 Cephalobranchiata, in the Schuylkill River, near Philadelphia. 

 The Nemertine worms, so common in the sea, have only one 

 representative in fresh water of certainly a very divergent form ; 

 of Sponges we find only one genus, Spongitta ; of the Hydroids 

 only two, Hydra and Cordylophora (fig. 40), which, in the course 

 of time, have become true fresh-water animals. 58 



FIG. 39, Oyster from the Cnmalaran River at Basilan (south of Mindanao) ; it lives in 

 spots where the water is quite fresh. 



C. The effect of the different percentage of salt in the 

 water. The cases adduced above prove that it is often impos- 

 sible to distinguish, by systematic characters alone, whethc 3r an 

 animal is fresh-water or marine, since there aro many special in 

 fresh water whose nearest allies live in the sea, and vice 

 Theoretically, then, we must admit that there is no g 

 and insuperable impossibility that they should exchange their 

 life in one medium for that in the other. But this theoretical 

 possibility is not, so far as we know, universally practical ; 

 for whole groups as the Brachiopoda, Sipunculidae, andli 

 Echinodermata have hitherto been found only in the sea. 

 The question now is : What causes have prevented or still pre- 

 vent a transfer of marine animals from sea-water to fresh 



