The Hydrozoa are a large group of solitary and colonial animals which have a complex life cycle which often involves an alternation between sessile (fixed in one place) polyp phase and a free-swimming medusa stage, similar to the True Jellyfish.
In some groups, either the polyp or the medusa stage may be missing. Some Hydrozoa families are able to produce calcium carbonate skeletons and form colonies, similar to the hard corals, however despite exhibiting very similar characteristics, these are not true corals.
Hydrozoa capture their prey and ward off enemies using stinging cells called nematocysts. Nematocysts are tiny cellular structures consisting of coiled, hollow and usually barbed, thread-like stingers, which usually contain poison.
The nematocysts are located throughout the tentacles that radiate either from the exterior in the sessile (stationary, fixed in one place) stage, or downward from the edge of the umbrella dome in the medusa (jellyfish) stage.
The reef building 'fire coral' family Milleporidae family can quickly grow large skeletons covered with many surface pores which contain stinging nematocysts.
Fire Coral species are sessile (stationary, fixed in one place) Medusozoa, which are capable of producing calcium carbonate skeletons and forming colonies, similar to the hard corals, however these are not true corals. Some Fire Corals species are important reef builders, with several species of Millepora able to very rapidly grow calcium carbonate skeletons.