THE ANIMALS OF LAKES AND RIVERS 191 



animals succeed in spreading from one part of the lake 

 to another if they have no free-swimming stage ? The 

 answer is apparently that the distribution is largely 

 passive, being assisted by sudden freshets, which may 

 produce currents of unusual violence, capable of carry- 

 ing adults, or, more likely, resting eggs to a distance. 

 In addition there are some curious special adaptations. 

 We may take two examples of famihar forms. The 

 fresh-water hydra as contrasted with the marine sea- 

 firs has no free-swimming stage. While the weather is 

 warm it reproduces itself by buds ; towards autumn 

 it produces coated eggs which survive the winter, and 

 produce fresh hydrae in spring. These sheUed eggs 

 are doubtless easily transported by currents. More 

 remarkable are the conditions in the fresh-water 

 mussel (Anodon). Here the mother retains the young 

 within her body till sticklebacks approach the mud in 

 which she is hving. She then discharges the young, 

 who fix themselves to the fish, and are thus carried 

 about for a time, and so distributed. Later they drop 

 from the stickleback to the mud, and develop into 

 mussels like the parent. 



With this prehminary account of the conditions of 

 Mfe in lakes and rivers, and of the resultant adapta- 

 tions, we may proceed to consider the animals which 

 constitute the faima. 



A considerable number of mammals show at least 

 a partial adaptation to fresh-water hfe. Thus we have 

 many kinds of otters, which have webbed feet and feed 

 upon fish ; not a few insectivores, such as the common 

 water-shrew and the Russian desman ; various rodents, 

 such as the beaver and the water-vole ; a monotreme, 

 the curious duck-mole or Omithorhjnichus of AustraHa, 

 and so on. In most of these cases the modifications are 



