The fisheries are all fresh-water, principally white-fish, pickerel and pike.
The sub-order includes the family Hydridae, containing the common fresh-water polyps of the genus Hydra.
This genus comprises fresh-water polyps of simple structure.
Polypodium hydriforme Ussow is a fresh-water form parasitic on the eggs of the sterlet.
Of doubtful position, but commonly referred to the Trachylinae, are the two genera of fresh-water medusae, Limnocodium and Limnocnida.