Such small creatures as Daphnia pulex, Cyclops quadricornis and Rotifera should be introduced into ponds.
Both Daphnia and Cyclops are bred in stagnant water in which there should be a good stock of weeds.
The recovery or death of the Daphnia depends therefore on the issue of the struggle.
The water-flea, Daphnia (a crustacean), lays two kinds of eggs known as “summer” and “winter” eggs.
I have already spoken of the fresh-water shrimp and the water-flea (Daphnia pulex).
Of Crustaceae: near the sea-shore a Pagurus existed in astonishing numbers, and in the sweet waters a Daphnia.
Some of these belong to the genus Daphnia, and differ from the species found in other situations by their glassy transparency.
Daphnia must be reared in a stagnant pond, the fresh-water shrimp (Gammarus pulex) in running water, with plenty of weeds.
Wolfgang Ostwald has produced experimentally, through variation of temperature, dimorphism of form in Daphnia.