These organisms may exist as either polyps or medusae. The majority of species are marine and colonial, but many species are solitary and some live in fresh water. Hydrozoans include such diverse forms as hydra, which are a group of species of solitary, fresh-water forms that live as polyps throughout their lives. Reproduction by hydras can be the typical form for polyps, asexual by budding, or by sexual means in which temporary gonads appear on the polyp itself. Hydrozoans also include colonial species, called hydroids.
Hydroids are actually colonies of polyps growing on a common stalk.
In this typical case, the medusae, which are usually free-swimming, leave the colony, mature, and produce gametes for sexual reproduction. The gametes are shed, fertilization takes place, and the zygote develops into a free-swimming, ciliated planula larva. After a time the planula settles to the bottom and develops into a polyp, which by budding eventually forms a new colony. Others, called hydrocorals, have calcareous skeletons that resemble those of true corals. Hydrozoan medusae all have a distinctive structure called a velum .