SUMMARY


  1. Members of the class Anthozoa are polypoid cnidarians; the medusoid stage is entirely lacking.
  2. The anthozoan polyp is more specialized than that of hydrozoans, and its cellular mesoglea, septate gastrovascular cavity, cnidocytes in gastric filaments, and gastrodermal gonads indicate a closer phylogenetic relationship with the Scyphozoa than with the Hydrozoa.
  3. The difference in the body form of the Scyphozoa and the Anthozoa (medusa versus polyp) may be reconciled if the anthozoans are derived through the polypoid larva of scyphozoans.
  4. The two subclasses, the Zoantharia and the Octocorallia, reflect different levels of structural evolution within the Anthozoa. The Octocorallia have retained an arrangement of eight complete mesenteries and eight tentacles, which may be the primitive anthozoan condition. Colonial organization is characteristic of almost all octocorallians, and the polyps are interconnected through a complex mass of mesoglea and gastrodermal tubes. The zoantharia display a more complex system of mesenteries, which always exceed eight in number. There are many solitary forms, and colonial species are connected by more or less simple outfoldings of the body wall.
  5. Sea anemones are the principal group of solitary anthozoans, and perhaps because of their solitary condition, many species have evolved a larger size than most other anthozoan polyps. The number and complexity of their mesenteries, providing a large surface area of gastric filaments, may be related to the utilization of larger prey.
  6. The majority of .anthozoans are colonial, and this type of organization has evolved independently a number of times within the class. Although colonies may reach a large size, the individual polyps are generally small. There are some groups with polymorphic colonies, but this condition is not as widespread as in the hydrozoans.
  7. Scleractinian corals, although similar to sea anemones, are largely colonial and are unique in their secretion of an external calcareous skeleton. The skeleton provides the colony with a uniform substrate on which the living colony rests. The sclerosepta may contribute to the adherence of the polyps within the thecal cups and provide some protection against grazing predators when the polyps are withdrawn.
  8. The majority of scleractinian corals are tropical reef inhabitants (hermatypic) and harbor zoo¬xanthellae. Zooxanthellae are found in many other anthozoans as well as some scyphomedusae and some hydrozoans.
  9. The colonial alcyonaceans, or soft corals, which are most abundant on Indo-Pacific reefs, in many ways parallel the scleractinian corals, for the massive coenenchymal mass forms the substrate from which the individual polyps arise.
  10. The branching, rodlike colonies or gorgonian corals are adapted for exploiting the vertical water column while using only a small area of the substrate for attachment. Flexible support is pro¬vided by a central, organic skeletal rod and separate calcareous spicules embedded in the coenenchyme.
  11. The pennatulaceans, which include sea pens, sea feathers, and sea pansies, are adapted for life on soft bottoms. A large, primary polyp, which determines the form of the colony, not only provides anchorage in the sand but also acts as the sub¬strate from which the small, secondary polyps arise.
  12. A planula larva is characteristic of most anthozoans and develops into the polyp. Colonial forms are derived by budding from the first polyp.