Hydrozoa are a taxonomic class

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater. Hydrozoans are related to jellyfish

Jellyfish

Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

 and corals and belong to the phylum Cnidaria

Cnidaria

Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

.

Some examples of hydrozoans are the Freshwater Jelly (Craspedacusta sowerbyi), the freshwater polyps (Hydra

Hydra (genus)

Hydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...

), Obelia

Obelia

Obelia is a genus in the class Hydrozoa, which consists of mainly marine and some freshwater animal species and have both the polyp and medusa stages in their life cycle...

, the Portuguese Man o' War

Portuguese Man o' War

The Portuguese Man o' War , also known as the Portuguese man-of-war, man-of-war, or bluebottle, is a jelly-like marine invertebrate of the family Physaliidae...

 (Physalia physalis), the chondrophore

Chondrophore

The chondrophores or porpitids are a small and very unusual group of hydrozoans today classified as family Porpitidae. Though it derives from an outdated name for this lineage , some still find the term "chondrophore" useful as a synonym to "porpitid" in discussions of the three genera contained...

s (Porpitidae), "air fern

Air fern

Neptune plant or Air fern is a name given to a product that is composed of a species of marine animal called Sertularia argentea, also known as the "sea fir"....

" (Sertularia argenta) and the pink-hearted hydroid

Tubularia

Tubularia is a genus of hydroids that appear to be furry pink tufts or balls at the end of long strings, thus causing them to be sometimes be called "pink-mouthed" or "pink-hearted" hydroids. T. larynx is described as:The stems are tubular, with a yellowish coloured tegument and are branched at the...

s (Tubularia).
Most hydrozoan species include both a hydroid

Polyp

A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...

 and a medusoid

Medusa (biology)

In biology, a medusa is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below...

 stage in their life cycle, although there are a number that have only one or the other.