Hydrozoans are small predatory animals. Most hydrozoans observed in the North-East Atlantic are hydroids. A hydroid has typically three life-cycle stages.
The first stage is a tiny pelagic larva. The larva develops into a second, often colonial, hydroid stage. The colony may be pelagic (free-swimming) or sessile (attached to a surface). Sessile colonies may be branched and the branches bear the individual animals (also called zooids or hydroid polyps). Many species form large, conspicuous colonies by asexual reproduction.
Finally converting into the third stage in their life-cycle, the polyps convert to radially symmetrical medusae. The medusae are released from the colony and become pelagic. The body in the medusoid stage take the shape of dome-like umbrella with tentacles along the edge. They are usually smaller than the more familiar, but similar jelly fishes. This is usually considered their adult stage, when reproduction is accomplished sexually. By utilizing both reproduction strategies the species secure a large number of off-springs as well as a necessary genetic variation.
The taxonomy of Hydrozoa is under frequent revision, last in 2009. My major sources on taxonomy all have different ways to classify hydrozoans. They seem to agree that most species are marine and that. All species presented here are, according to MarBEF, classified as member of the subclass Hydroidolina, while the Marine Species Identification Portal classify them as member of the class Leptolida (of the superclass Hydrozoa). Regardless, three important groups or orders, according to MarBEF, dominate the Norwegian marine fauna: