Hydrozoans are a diverse group of small, mostly marine animals belonging to the phylum Cnidaria. They are characterized by their simple body structure, which typically includes a polyp stage and a medusa stage. Hydrozoans can be solitary or colonial, forming large groups that can float or drift in the water. These creatures are known for their stinging cells, called nematocysts, which they use for capturing prey and defense. Hydrozoans can be found in various marine environments, from shallow coastal waters to the deep sea, and they play important roles in the aquatic food web.