Hydrozoa is a class 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, in some species, a medusa stage. Hydrozoans can be solitary or colonial, forming intricate structures that can float or attach to surfaces. These creatures are known for their stinging cells, called nematocysts, which they use for capturing prey and defense. Hydrozoa includes well-known species like Portuguese man o' war, which is a colonial organism made up of specialized polyps and medusae working together.