Class Anthozoa includes Cnidaria that have no jellyfish stage. This is the largest class of cnidarians, containing over 6,000 species. A gullet extends for a short distance into the gastrovascular cavity, and septa are present, which increase the surface for digestion and absorption. Anthozoa are flower animals, including a great many beautiful and colorful organisms, e.g., the sea anemone, sea pansy, sea fan, and coral. Anthozoans are colonial or solitary organisms.Subclass Alcyonaria
Subclass Alcyonaria includes almost universally colonial organisms in which each of the polyps, or hydroid members, has eight feathery tentacles. Most of them produce a skeleton, and many make some contributions to coral reefs. While some are found in temperate seas, they are especially common in subtropical to tropical regions. The organ pipe coral ( Tubipora ), a soft coral ( Alcyonium ), the Indo-Pacific blue coral ( Heliopora ), and the sea pens, which have a stalk extending into the bottom mud or sand, are some typical alcyonarian corals. Horny corals, of the order Gorgonacea, are perhaps the best known. These form branching, upright colonies and have a skeleton that is partly composed of a horny material called gorgonin. These are the sea whips and sea fans so characteristic of shallow tropical waters.
The subclass Zoantharia includes both solitary and colonial forms, in which the polyp has more than eight tentacles. The solitary sea anemones belong here, in the order Actiniaria, characterized by the lack of a skeleton. The stony corals so important in forming coral reefs belong to the order Madreporaria; they are especially characterized by their calcium carbonate exoskeleton, marked by many cups for the polyps, each of which contains stony septa dividing the gastrovascular cavity into compartments. The shape of coral skeletons depends on the pattern of growth of the colony. For example, in brain corals the polyps are arranged linearly; in the eyed coral ( Oculina ) the polyps are separated from each other by spaces, giving the skeleton a pitted appearance. The burrowing anemone, Cerianthus, lives in burrows in the sand and has a greatly elongated body. It is characteristic of the order Ceriantharia.
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