Hydrozoa (phylum Coelenterata, class Hydrozoa) are coelenterates that are closely related to the hydra, sea anemones, corals, and jellyfish. Although a large majority of these species are common and widespread, they are often overlooked, as they are all small animals. The vast majority are marine species, but several freshwater hydrozoans have also been identified, for example, Cordylophora lacustris and Craspedacusta sowerbyi.
There is considerable variation in the structure and appearance of hydrozoans. All species, however, have a stalk-like arrangement known as a polyp, which bears a number of tentacles, as well as the mouth. In addition, two main types of hydrozoa exist: a polypoid structure which is sessile, remaining in the same place, and a medusoid form which is free-swimming. Many polypoid hydrozoans, however, may have a medusoid larval phase which eventually settles onto some substrate. Some species are solitary, but the majority are colonial. In the latter, the colony arises from a single basal root which rests on the substrate and from which individual polyps arise. Both colonial and individual species lack a hard outer skeleton.
A special feature among colonial species is the presence of individual polyps that fulfil separate roles. Some polyps are, for example, specialized for feeding (gastrozooids), while others are responsible for reproduction (gonozooids) or defense. While most colonial sessile species are small and feed by filtering zooplankton from the surrounding water currents, some of the medusoid forms are quite large and capable of feeding on small fish. Floating pelagic species such as Porpita and Velella, which resemble small jellyfish, may reach 1.5-2.5 in (4-6 cm) in diameter. These are colonial species made up of large numbers of gastrozooids and gonozooids; the body is modified into a flattened structure with a float on the upper surface to provide buoyancy. Some species, such as Velella, or the by-the-wind-sailor, as it is commonly known, have an additional small "sail" on the upper surface to catch the wind and assist further with dispersal.