The coelenterates are multicelled animals which, though more complex
than the sponges, are rather primitive animals. The living animal is
characterized by a sac-like body cavity, a definite mouth, and tentacles
which bear stinging cells. Some forms, for example, the jellyfishes,
have an umbrella-shaped body and are single free-moving organisms.
Others, like the colonial corals, are composed of many individuals
living together in a colony.
Most zoologists and paleontologists recognize three classes of
coelenterates: (1) the Hydrozoa, containing the small animals known as
hydroids, (2) the Scyphozoa, which includes the jellyfish, and (3) the
Anthozoa, which includes the corals and sea anemones. Because of their
extreme fragility and lack of hard parts, hydrozoans and scyphozoans are
not commonly found as fossils. They do, however, have a long geologic
history and may be preserved when unusual conditions of fossilization
occur. The anthozoans, especially the corals, are by far the most
important class geologically, and these forms have left a very good
paleontological record.