The coelenterates are a large and beautiful group of aquatic organisms. Their adult form is generally radially symmetrical; that is, their body parts are arranged around a central axis. The brilliant coloring of many species combined with a radial symmetry, often creates a beauty that is surpassed by few other animals. The radial symmetry is commonly considered justification for uniting the coelenterates and the related ctenophores within a division of phyla of the Animal Kingdom called the Radiata. The basic body plan is a simple one: the animal is essentially a hollow container, which may be either, vase-shaped, the polyp, or bowl-shaped, the medusa. The polyp is usually sessile; the medusa – motile. Both consist of two layers of tissue: ectoderm and endoderm. Between the two layers is a gelatinous filling, the mesoglea (“middle jelly”), which is made of a collagen-like material. The mesoglea of a polyp is thin. In a medusa, however, the mesoglea often makes up the major part of the body substance. Compared with the hydra, the jellyfish is upside down. Instead of the tentacles and mouth being up, as in the hydra, the tentacles and mouth are down.
One distinctive feature of the animals in this phylum is the coelenterons, a digestive cavity with only one opening. The Coelenterates possesses two basic metazoan structural features. There is an internal space of digestion called in coelenterates a gastro-vascular cavity. This cavity lies along the polar axis of the animal and opens to the outside at one end to form a mouth. The presence of a mouth and digestive cavity permits the use of a much greater range of a food sizes than is possible in the protozoan and sponges. Within this cavity, enzymes are released that break down food, partially digesting it extracellularly, as our own food is digested within the stomach and intestinal tract.
The second distinctive feature is the endoblast. Coelenterates are carnivores. They capture their prey by means of tentacles that form a circle around the “mouth”. These tentacles are armed with endoblasts; special cells that contain nematocysts (thread capsules). Nematocysts are discharged in response to chemical stimulus or touch. The toxin apparently produces paralysis substance attacking the lipoproteins of the nerve cell membrane of the prey. The largest jellyfish is Cyanea, whose tentacles may be over 30 meters long.
Coelenterates live singly or in colonies.
The phylum Coelenterate includes the familiar hydras, jellyfish, sea anemones, and corals.