:  The Hydrocorals
Hydrozoa: Hydrocorals
Body Form: The hydroid body is a 'polyp' - a tubular body with tentacles attached around an opening at one end that serves as the mouth. Stinging cells called nematocysts are used to paralyze and catch prey. The body wall is thin and lacks signficant mesoglea - the fluid between the inner and outer body wall. Their gut lacks the infolding of other Cnidarian body forms.
Natural History: The hydrozoans include hydras, fire corals, lace corals, and the famous floating Portuguese Man o' War (Physalia physalis) . The fire corals are one of the few non-stony corals that significantly contribute to reef-building. The hydrozoans include both solitary and colonial forms. They reproduce sexually or asexually. Sexual reproduction involves a free-swimming medusa stage that migrates to new areas, dispersing offspring away from parents. Some hydrozoans deliver very potent stings. All species are predatory and sting small fish, crustaceans, or other planktonic form. The sting paralyzes the prey, which is then drawn into the gut and digested. Some hydrocorals use zooxanthellae (Millepora) to supply well over half of their food needs, gaining the remainder from microplankton in the water. Other genera ( Distichopora, Stylaster) are nonphotosynthetic and supply their entire nutrition from microplankton. The fire corals are largely extinct and represented by only one family the Milleporidae. The Millepora often appear fuzzy because their numerous small tentacles cover their skeleton. They produce a potent proteinaceous toxin that is painful, hence the name "fire" coral. They are more closely related to hydroids and anemones than other corals. They prefer high current and light and use light for photosynthesis.
Lace Corals