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.
"Coelenterates" are invertebrate animals that include the hydras, jellyfishes, 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.