Class: Anthozoa

Anemones, corals, sea fans, and sea pens.

    Anthozoans are among the most complex of Cnidarians, and generally resemble an upside-down jellyfish in structure. Corals are tiny sessile animals that live in huge colonies and secrete a calcium-rich shell that forms reefs. Anemones grow much larger, are mobile plant-like animals that are common in intertidal zones. as a consequence, they bring the thin, delicate tentacles into the thick central body. In this state the anemones may stay out of the water for several hours with no ill effects. The flower-like anatomy of these animals gives the class its name: "Anthozoa" translates into "plant animals."

Shown are two green sea anemones. At right is a relaxed animal with open tentacles. The greenish blob in the lower left is a "closed" anemone. Photographed at the Fitzgerald Marine Reserve at San Mateo County, California, by Dr. Robert Sprackland.

    The internal structure of anemones is made of a hollow siphonoglyph, which serves as a mouth and a gastrodermis or stomach-like region for digestion. There is no anus, so the siphonoglyph doubles as mouth and anus.  There is also a series of baffled chambers made of septal muscles, providing hydrostatic support for the animal's body.

    All anthozoans possess stinging cells (cnidocytes) on their tentacles. There is an interesting case of mutualistic symbiosis¹ between some anemones and so-called anemone fishes. A group of brightly colored fishes (genus Amphiprion) live in very close proximity to anemones, getting protection from the cnidarians. Researchers learned that the fish are protected from the anemone's stinging cells because they rub themselves against non-stinging portions of the anemone. The resulting protein coating acts as a self-recognition mechanism--in essence, the anemone won't sting "itself." However, the anemone fishes do attract other fish species which become prey to the cnidarian.   
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¹ Mutualism is the state that occurs when two or more species live together and both (or all) species achieve some benefit and none are harmed by the other..
  Symbiosis is the general term for more than one species living together in some way that affects both (or all) the species involved. "Symbiosis" is simply Greek for "living together."