o stock with
creatures suitable for food, according to what he finds in the water.

Fresh-water snails are always desirable. In streams, or in ponds with
streams running into them, the fresh-water shrimps (_Gammarus pulex_)
should always be tried. It does not do in some waters, but where it does
thrive it increases very rapidly, and forms about the best article of
food that can be given to trout. _Corixae_, which thrive in ponds and
sluggish waters, should always be introduced. They increase rapidly, and
are taken by most fish, particularly by trout. The amateur should be
careful when he introduces these creatures to make sure that he is
putting in the right creature. The water-boatman (_Nautonecta glauca_)
is a member of the same family, but is no use as food for the fish. He
swims on his back, is longer and narrower than are _Corixae_, which do
not swim on their backs, are smaller, broader, and live much more under
water than the water-boatman. It is generally advisable to avoid
water-beetles, as most of them are more likely to do harm than good,
such a number of our water-beetles being carnivorous. They will probably
not harm adult fish, but they will destroy ova and fry. I have known a
_Dytiscus marginalis_ kill a trout of nearly a quarter of a pound in
weight.

In order to make sure of not introducing carnivorous water-beetles into
a water, I think it best as a rule not to introduce beetles at all.
_Corixae_ are, however, so like beetles, that many people call them
beetles, and therefore I will give a few points which will make them
easily distinguishable from each other. In beetles, the wing-cases
(elytra) meet exactly in the middle line, in _Corixae_ and other
water-bugs, the anterior wings, which resemble the elytra of beetles,
overlap, which causes the line on the back to curve away to one side at
the lower end. In beetles the wings which lie under the wing-cases are
folded up on themselves, and when spread out are much larger than the
wing-cases. The wings are transparent and very delicate. In _Corixae_ the
posterior wings, which lie under the hard and horny anterior wings, are
a little shorter than the anterior wings; they are not folded up on
themselves and are not so delicate and transparent as the wings of the
beetle.

Such small creatures as _Daphnia pulex_, _Cyclops quadricornis_ and
_Rotifera_ should be introduced into ponds.

Snails (_Gasteropoda_) may be roughly divided into three classes,
accord