Hydrozoans

Hydrozoa (hydrozoans; from Ancient Greek ὕδωρ (húdōr) 'water' and ζώα (zóa) 'animals'), also called hydroids, are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in freshwater habitats. Hydrozoans are related to jellyfish and corals, which also belong to the phylum Cnidaria.

Some examples of hydrozoans are the freshwater jelly (Craspedacusta sowerbii), freshwater polyps (Hydra), Obelia, Portuguese man o' war (Physalia physalis), chondrophores (Porpitidae), and pink-hearted hydroids (Tubularia).