This diverse invertebrate (invertebrates are animals without spinal columns) group includes corals, sea anemones, hydras, jellyfishes, and their relatives. All cnidarians are radially symmetrical (the body is symmetrical around a central axis), lack a head, usually have a crown of tentacles around the mouth, and possess nematocysts. About 9,000 living species are known.

Anthozoans include corals, sea anemones, sea pens, and sea pansies. These animals are either solitary or colonial polyps that live attached to a substrate (surface). Of the 6,000 known anthozoan species, corals comprise about 2,500 species.

The Class Anthozoa is further divided into three subclasses: Octocorallia, Zoantharia, and Tabulata (extinct colonial corals).

In this resource, the term "corals", refers to both Octocorallians and Zoantharians unless otherwise noted.

For a list of Anthozoan orders in their subclasses, see the appendix.

Fire or stinging coral is not a true coral. It is a hydrocoral (Class Hydrozoa). Hydrocorals are not discussed in this booklet.

The earliest reefs developed two billion years ago in the mid- to late Precambrian era. These reefs were built by colonies of calcareous algae, not corals.

Corals, sponges, bryozoans, and calcareous algae enhanced the growing reef community in the Paleozoic era, 245 to 570 million years ago. During this era, natural environmental changes led to periodic reef demise.

Hard corals developed into the prominent reef builders during the Mesozoic era, 65 to 245 million years ago. Coral reefs flourished until a devastating demise at the end of the era, when many coral families disappeared.

The species of corals that made up the reefs of the Tertiary period, 2 to 65 million years ago, were similar to today's species.