Hydrozoan Cnidarians gallery

Hydrozoans are Cnidarians which generally alternate between a fixed phase (polyp) and a free phase (jellyfish). They can be solitary or colonial. The majority of species are marine but there are also freshwater species (hydra). Cnidarians have a single orifice, the mouth, which is surrounded by tentacles. They are radially symmetrical animals. They are characterized by the presence of stinging cells, cnidoblasts, which allow them to neutralize their prey, hence their name (from the Greek cnid- = nettle).