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A class of the phylum Coelenterata. These organisms are marine, solitary or colonial, and exclusively polypoid coelenterates with no traces of a medusoid stage. Most anthozoans live attached to some firm object of the shore or on the sea bottom; some embed in the soft sediment. Anthozoans have a cylindrical body with an oral disk, mouth, stomodeum, hollow tentacles, endodermal gonad, and cellular mesoglea. The gastrovascular cavity is partitioned longitudinally into radial compartments by endodermal mesenteries or septa whose free edges, particularly, thicken and differentiate into mesenteric or septal filaments. The nervous system is a diffuse network of scattered nerve cells over the ectoderm and the endoderm. No localized sense organs are present.
Both sexual and asexual reproduction occurs. The germ cells are derived from the endoderm, and fertilization occurs either in the female gastrovascular cavity or in the sea. The zygote develops into either a ciliated swimming larva, the planula, or a young polyp.
The class Anthozoa includes the soft, horny, stony, and black corals, the sea pens, and sea anemones. The horny corals include the sea fans, sea whips, and sea feathers. The Anthozoa may be classified as listed here.
- Class Anthozoa
- Subclass Alcyonaria (Octocorallia)
- Order: Stolonifera
- Subclass Zoantharia (Hexacorallia)
- Order: Actiniaria
- Scleractinia (Madreporaria)
All anthozoans are marine and most are sedentary, except the free-swimming larval stages, while actinians, cerianthids, and pennatulans are somewhat movable. They are widely distributed over the world, extending from the Arctic to the Antarctic; however, they predominate in the tropic and subtropic areas of the Indo-Pacific Ocean. Actinians also inhabit colder water areas from which deep-sea species of gorgonians, pennatulans, and scleractinians have been collected.
Anthozoans seldom tolerate desiccation or heavy sedimentation. They are so sensitive to reduced salinity that they usually do not live near coastal areas where there is river drainage. Tropical corals are able to endure high temperatures and are adversely affected by low temperatures. Therefore, coral reefs are commonly located in tropic and subtropic regions. See Coelenterata, Hydrozoa
a class of invertebrates of the phylum Coelenterata.
Anthozoa are colonial or, more rarely, solitary marine organisms. The body of a separate individual is radially symmetrical. The polyp is usually cylindrical; its base is either fused with the colony or, in solitary forms, has a stalk that attaches it to the bottom. At the opposite end of the body is a mouth surrounded by tentacles. From it begins the ectodermal gullet, leading to the gastrovascular cavity, which is divided by incomplete partitions into chambers. The body walls consist of ectoderm and endoderm, with a mesogloeal interlayer between them. The nervous system consists of nerve cells, scattered mainly in the ectoderm. Anthozoa reproduce sexually and asexually. The sexual products develop in the endoderm of the septa. The offspring usually leave the maternal organism in the larval stage, the planula, swim for some time, and then settle on the bottom, where they attach themselves and become adult polyps. Asexual reproduction occurs by budding. Anthozoa form colonies as a result of incomplete budding.
There are two extant subclasses: Octocorallia (or Alcyonaria) and Hexacorallia (or Zoantharia). The Octocorallia consist of colonial forms whose polyps have eight feathery tentacles. In some cases the axial skeleton of the colony consists of a horny substance (for example, in Gorgonaria). Representatives include red coral, organ-pipe coral, and sea feather. In Hexacorallia the number of tentacles is usually a multiple of six. Among the Hexacorallia are solitary, askeletal sea anemones (Actiniaria), which often enter symbiotic relationships with animals like hermit crabs and crabs, and madrepores, which have a highly developed calcareous skeleton (they form coral reefs in tropical seas). There are approximately 6,000 species, found in all seas with sufficiently high salinity. In the northern and Far-Eastern seas of the USSR there are about 150 species.
Representatives of the most primitive Anthozoa (subclass Tabulata) were first found in Cambrian deposits. Rugosa and Heliolitoidea appeared in the Middle Ordovician or somewhat earlier. About 80 families of now-extinct Anthozoa existed in the Paleozoic. On the border between the Paleozoic and the Mesozoic, Tabulata and Rugosa became almost completely extinct; representatives of the subclasses Hexacorallia and Octocorallia appeared at the beginning of the Mesozoic. Limestones formed by fossil corals as well as some extant madrepores are used in the manufacture of building materials; red coral is used in making jewelry.
D. V. NAUMOV