The animal of the genus Paludina is an inhabitant of fresh waters: it
takes its specific name from the fact of the young being hatched within
the parent shell, and deposited in the waters perfectly formed. In
nearly the whole of this class the eggs are laid either in water or in
the earth, and afterwards hatched after a longer or shorter period.
[Illustration: _Planorbis corneus. Paludina vivipara. Lymnæa
stagnalis._]
These shells are found generally in running streams of fresh water;
sometimes, however, they are met with in brackish water, at the
mouths of rivers. The operculum of the Paludina is of a strong horny
substance. The young, immediately they are hatched, attach themselves
to the outside of the shell of the mother, where they remain until they
are sufficiently strong to trust themselves in the water.