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A large class of sedentary marine coelenterates which includes sea anemones and corals; the medusoid phase is entirely suppressed.

Coelenterates are invertebrate animals that include the hydras, jellyfish, sea anemones, and corals, all of which are characterized by a single internal cavity serving for digestion, excretion, and other functions and having tentacles on the oral (mouth) end.

Medusoid refers to the tentacled, usually bell-shaped, free-swimming sexual stage in the life cycle of a coelenterate; such as, a jellyfish.

actinozoa, anthozoa
Solitary or colonial coelenterates without any free-swimming medusa stage and with the coelenteron divided by mesenteries.

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.

—A.W. Leftwich, A Dictionary of Zoology;
Crane Russak & Company; New York; 1964.