The group includes most of the common sea-anemones and corals.
Coelenterata consist of a rather large phylum of sedentary or free-swimming animals having a radially symmetrical sac-like body with a mouth at one end, usually surrounded by a ring of stinging tentacles.
The digestive cavity, known as the coelenteron, has only this opening, but repeated budding may give rise to a colony in which all the body-cavities are connected with each other.
The body-wall is diploblastic, that is, made of two layers of cells, ectoderm and endoderm with a gelatinous layer known as the mesogloea between them.
The individuals of a colony are known as polyps or hydroids. They form the asexual sedentary generation which alternates with a free-swimming sexual phase known as the medusa. The latter is a disc-shaped jelly-fish which breaks away from the parent colony and swims away. It produces gametes and after fertilization has taken place, a new polyp colony develops.
Included in the Coelenterata are corals and anemones in which the hydroid stage is dominant, jelly-fish in which the medusoid stage is dominant, and complex forms such as the Portuguese Man-of-war formed by a large floating colony of diverse polyps.