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Phylum Cnidaria Hydrozoan (hydroid) polyp

Cnidarians (also known as Coelenterates ) are common or abundant in many marine environments. They display a wide variety of body forms and lifestyles, and include such familiar organisms as jellyfish , corals , and sea anemones. Throughout the phylum there is typically an alternation of two body forms, the polyp and medusae , however, in some groups one or the other form is reduced or absent. There are approximately 9,000 living species of Cnidarians , and they can be divided into four classes: Hydrozoa Scyphozoa Anthozoa Cubozoa 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. 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).

Scyphozoan jellyfish, from Huxley, 1878.

Sea anemone, (Actinia equina ), class Anthozoa , from Halstead , 1965.

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.

A branch of soft coral (Class Anthozoa), photo by T. Popin.

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 .

Cubozoan medusae

modified from BIODIDAC, Univ. of Ottowa

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 ). The general Characteristics of the Cnidaria are: Radially symmetrical, sometimes with biradial symmetry, and an oral-aboral axis . Diploblastic , having ectoderm and entoderm , with a mesoglea or mesenchyme layer between. Digestive cavity with only one opening, and which is also the only body cavity . Have unique structures, cnidae , which are stinging or adhesive structures produced by cells called cnidocytes . No specialized circulatory , respiratory, or excretory systems. Basic condition of alternating polypoid and medusoid body forms, but many forms have reduced or lost one or the other of these two forms. A diffuse nervous system as a simple nerve net. Usually have a ciliated planula larvae. REFERENCES Barnes, 1980 BIODIDAC, Univ. of Ottowa Brusca & Brusca, 1990 Barnes, Calow, and Olive, 1993 Halstead, 1965 Meglitsch & Schram, 1991

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