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, according to the shape of their shells: (1) Flat-shaped coils (type _Planorbis corneus_); (2) Oblong-shaped, somewhat like a trumpet (type _Limnaea stagnalis_); and (3) Ear-shaped (type _Limnaea auricularia_). _Limnaea auricularia_ is particularly suitable for deep waters, and _L. pereger_, whose shell is of type 2, is a most valuable addition to the food supply in any fish pond. It is one of the commonest of our fresh-water snails. Mussels (_Conchifera_) are another valuable article of food. There are a great many different kinds, and the larger ones should, as a rule, be avoided. _Sphaeriidae_ and _Pisidia_ are probably the best. In many cases it is advisable to attempt the introduction of some flies which are not present. There are several cases in which the May-fly has been successfully introduced, and also the Grannom. Small _Ephemeridae_ seem to me preferable to any other flies. With regard to suitable plants for comparatively deep water in ponds or lakes, lakewort and stonewort grow on the bottom, and do not, as a rule, attain any considerable height. White and yellow water-lilies also grow in fairly deep water; the water-lobelia is also an excellent plant for ponds. In streams some of the best plants are water-crowfoot, water-starwort, and the great water moss. Anacharis should not be introduced into any water, either pond or stream, unless it can be kept down easily. It will otherwise become an unmitigated nuisance. Marginal plants are a very important consideration, and plenty of them should be grown. Water-celery and water-cress are perhaps the best food-producing marginal plants that can be grown. Bullrushes and brooklime are also good, but the bullrushes must be planted judiciously. CHAPTER III SUITABLE FISH AND SUITABLE WATERS