Of the whole earth's surface the freshwaters form a very small fraction,
about a hundredth, but they make up for their smallness by their
variety. We think of deep lake and shallow pond, of the great river and
the purling brook, of lagoon and swamp, and more besides. There is a
striking resemblance in the animal population of widely separated
freshwater basins: and this is partly because birds carry many small
creatures on their muddy feet from one water-shed to another; partly
because some of the freshwater animals are descended from types which
make their way from the sea and the seashore through estuaries and
marshes, and only certain kinds of constitution could survive the
migration; and partly because some lakes are landlocked dwindling relics
of ancient seas, and similar forms again would survive the change.
A typical assemblage of freshwater animals would include many Protozoa,
like Amoebæ and the Bell-Animalcules, a representative of one family
of sponges (Spongillidæ), the common Hydra, many unsegmented worms
(notably Planarians and Nematodes), many Annelids related to the
earthworms, many crustaceans, insects, and mites, many bivalves and
snails, various fishes, a newt or two, perhaps a little mud-turtle or in
warm countries a huge Crocodilian, various interesting birds like the
water-ouzel or dipper, and mammals like the water-vole and the
water-shrew.
Freshwater animals have to face certain difficulties, the greatest of
which are drought, frost, and being washed away in times of flood.
There is no more interesting study in the world than an inquiry into the
adaptations by which freshwater animals overcome the difficulties of the
situation. We cannot give more than a few illustrations.
(1) Drought is circumvented by the capacity that many freshwater animals
have of lying low and saying nothing. Thus the African mudfish may spend
half the year encased in the mud, and many minute crustaceans can
survive being dried up for years. (2) Escape from the danger of being
frozen hard in the pool is largely due to the almost unique property of
water that it expands as it approaches the freezing-point. Thus the
colder water rises to the surface and forms or adds to the protecting
blanket of ice. The warmer water remains unfrozen at the bottom, and the
animals live on. (3) The risk of being washed away, e.g. to the sea, is
lessened by all sorts of gripping, grappling, and anchoring structures,
and by shortening the juvenile stages when the risks are greatest.