190 THE ANIMALS OF LAKES AND RIVERS 



swimmiag larvae called planulae ; Hydromedusae 

 produce free-swimming medusoids ; polychaete worms 

 have free-swimming larvae called trochospheres, all 

 delicate forms incapable of swimming against currents. 

 These particular ponds have at intervals a connexion 

 with the sea, and so can be peopled afresh from it, but 

 if we suppose that at some future period the connexion 

 with the sea were to be reduced to a swift-flowing 

 stream, coidd we suppose that the animals would 

 persist? Obviously not, for the medusoids, the 

 planulae and the trochospheres would tend to be swept 

 out to sea by the stream, and there would be thus no 

 young forms to replace the parents when death took 

 place. So real is this danger that, with rare excep- 

 tions, fresh-water organisms have no free-swimming 

 stage in their life-history. Only those marine animals 

 could colonize fresh water which had no plankton 

 stage in their life-history, or which managed somehow 

 to survive until this free-hving stage could be sup- 

 pressed by progressive variation. This perhaps gives 

 us a second reason why so many animals with a 

 terrestrial ancestry have succeeded in colonizing fresh 

 water, for these forms would not have a free-swimming 

 stage. Thus while most marine gasteropods have free- 

 swimming larvae, fresh-water snails, like land snails, 

 hatch as miniature adults, which makes life in ponds 

 and streams easy for them. These arguments of course 

 only apply to those dehcate organisms which are in- 

 capable of swimming against currents. Relatively 

 powerful swimmers, Uke many insect larvae, can 

 resist moderate ciurents, though they avoid regions 

 where the water is in rapid movement. 



But, it may be said, in a large lake the need for dis- 

 tribution is as great as in the sea, how do the lake 



