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 familiar 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 shelled 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 living. 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 preliminary account of the conditions of 
life in lakes and rivers, and of the resultant adapta- 
tions, we may proceed to consider the animals which 
constitute the fauna. 
A considerable number of mammals show at least 
a partial adaptation to fresh-water life. 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 Ornithorhynchus of Australia, 
and soon. In most of these cases the modifications are 
