Anthozoa

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The Class Anthozoa includes a variety of animals that have polyps with a flower-like appearance. In these forms, the gastrovascular cavity is large. It is divided by walls or septa, which arise as folds from the body wall. These folds, along with the mouth and pharynx, are usually arranged in a biradially symmetric pattern.

Anthozoans include sea anemones, a variety of corals, sea fans, and sea pens. Sea anemones are carnivorous polyps that are quite large, ranging up to 200mm in length. They tend to be brightly colored. Most species live in warm water. They feed on fishes, which are caught by means of the numerous nematocysts in their tentacles. These animals are known for their symbionts. These include species of fish that actually live among the tentacles of large anemones, somehow avoiding lethal contact with the nematocysts. Other anemones have unicellular algae living within their tissues, from which they probably derive some nutrition. Yet others have a symbiotic relationship with hermit crabs, which gather up the anemones and place them on the snail shells that the crabs occupy. The anemones benefit from food particles dropped by the crab, and the crab gains protection from predators due to the presence of the nematocyst-laden anemones.

The Class Anthozoa also includes many kinds of corals, including many reef-building species. Reefs are formed by the calcareous skeletons of many generations of coral polyps. The polyps inhabit only the surface of the reefs. These reefs are among the most productive environments of the world, housing thousands of species of fish and invertebrates, not to mention plants and protists. Like some anemones, many corals are inhabited by symbiotic algae called zooxanthellae. These photosynthetic algae are essential for those coral, which generally do not live at depths to which light does not penetrate.

Contributors

Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor, John B. Burch (author), Mollusk Division, Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).