Hydrozoans are mostly marine species, which form colonies of individuals. During the course of their life cycle, the specimens go through an asexual phase in the form of benthic polyp, and later a reproductive phase in the form of planktonic jellyfish.
During the benthic phase, the polyps can generate colonies of a few centimetres in length, and which are usually characterised by a chitinous cover, called the perisarch. In these colonies, the different individuals usually have very different functions, such as defence, reproduction, feeding, ... In relation to the jellyfish phase, they are usually small (between 0.5 and 6 cm) and not very striking, hence they are probably the least known group of Cnidarians. Jellyfish are characterized by having 4 tentacles or multiples of 4, with which they are able to capture their food, beeing even small fishes. It should also be noted that they may have ocelli on the top of the bell, allowing them to have some notion of the brightness of the environment.
Approximately 3,000 species belongs to the group of Hydrozoans have been described, the majority of which are marine species, although this is the only group of Cnidarians in which it is possible to find freshwater species, such as hydras.