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CNIDARIA (Corals, Jellyfish, Sea Anemones)
The Cnidaria are primarily represented in the fossil record by the corals. A few fossils interpreted as jellyfish have been found, but the corals have the external skeleton necessary for common fossilization. Corals have a hard, calcareous skeleton, and may be solitary or colonial. We tend to think of corals as been colonial with small individuals, but many Paleozoic species are fairly large and solitary such as the horn coral in the picture on the left. Colonial corals are composed of many polyps living together. The skeletal parts formed by polyps are called corallites. Each corallite is a small (several millimeters to several centimeters in diameter), roughly circular or hexagonal opening, with internal radial partitions called septae in the Rugose and Scleractinian corals. Tabulate corals lack septae. Corals are represented in the fossil record from the late Precambrian (Proterozoic) to Recent for the Cnidaria and the Cambrian to the present for the corals.
CLASS ANTHOZOA (corals and sea anemones)
SUBCLASS RUGOSA
Most rugose corals are solitary and conical (shaped like cones) and septae are visible in the circular opening of the cone. The picture on the left is a rugose coral. A few rugose corals are colonial, having hexagonal corallites with septae The rugose corals are all extinct and lived between the Cambrian and the Permian.