HYDROMEDUSAE, a group of marine animals, recognized as belonging to the Hydrozoa by the following characters.
The Hydromedusae represent, therefore, a sub-class of the Hydrozoa.
The Hydromedusae contrast with the Scyphomedusae in the following points.
The Hydromedusae form a widespread, dominant and highly differentiated group of animals, typically marine, and found in all seas and in all zones of marine life.
In fresh-water Hydromedusae the life-cycle is usually secondarily simplified, but in marine forms the life-cycle may be extremely complicated, and a given species often passes in the course of its history through widely different forms adapted to different habitats and modes of life.