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The class Hydrozoa includes hydroids and hydromedusae , as well as the familiar freshwater Hydra . Hydroids superficially resemble plants or "feathers" to those unfamiliar with them, although there are a wide variety of body forms. Often colonial, many common hydroids have colonial polyps that are interconnected by stalk-like structures. There are about 2,700 living species of hydrozoans , including a number of freshwater forms.
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The Class Scyphozoa consists of organisms popularly known as jellyfish , which is the medusoid body form of this class. The polypoid stage is typically reduced or absent. Scyphozoan medusae are usually planktonic , although capable of some locomotion, but a few forms are benthic or attached. About 200 species of Scyphozoans are found in the marine environment only (there are no freshwater forms).
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Chrysaora sp., a Scyphozoan jellyfish, modified from Johnson and Snook, 1927
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Sea anemone, (Actinia equina ), class Anthozoa , from Halstead , 1965.
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The class Anthozoa includes the forms commonly known as sea anemones, corals , sea fans, and sea pens . There are both solitary and colonial forms that have no medusoid stage. Most are benthic and sessile, although some forms are capable of limited locomotion. The polyps reproduce both sexually and asexually . There are about 6,000 species of Anthozoans , all are marine.
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Corals are colonial anthozoans , and in tropical waters may be the dominant reef-building organisms. Many corals , however, do not take part in reef formation, and some forms are solitary polyps .
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A branch of soft coral (Class Anthozoa), photo by T. Popin.
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Cubozoan medusae modified from BIODIDAC, Univ. of Ottowa
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The Class Cubozoa are popularly known as "box jellyfish " or sea wasps, and include some of the most venomous marine organisms known. Relatively small and transparent, the medusa has a four sided "square" appearance. Although Cubozoans are found throughout most tropical regions, they are especially common in the Indo-West Pacific, and at certain times cause many problems for swimmers (including fatalities) along the Australian coast (especially off Queensland beaches ).
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