The cnidarian body is radially symmetrical, consisting of a tube or sac with one opening, and a mouth surrounded by tentacles. The wall has an epidermis and gastrodermis, separated by a middle layer which can be either very thick or quite thin. A distinctive characteristic of cnidarians is nematocysts. They provide effective means of snaring prey and protection. It is a capsule with a long thread forcefully everted when triggered by contact with prey or predators. They are constantly produced by the cnidarian. Humans in contact with certain cnidarians may have effects such as rashes or in rare cases death.

Cnidarian may either be in polyp or medusa form. Polyps are tubes with mouths surrounded by tentacles made for sedentary life. Medusas float free in water and swim in pulsating contractions. Some life cycles include them both. Both sexual and asexual reproduction by longitudinal splitting or regeneration can be found here.

Anthozoa includes the soft corals, sea anemones, and the stony corals. Polyps, no known medusae. Coelenteron divided into chambers by septa which extend from the body wall toward the center. Mouth is situated on an oral disk surrounded by tentacles. The gastrodermis contains nematocysts and gonads, with the sexes generally separate. Some species are oviparous, shedding their eggs into the water. Others are viviparous, with fertilization taking place in the body of the female. The egg developed into a minute planula larva. Some brood their young beyond the planula stage and release them as polyps. Except for sea anemones, nearly all anthozoans are colonial and undergo budding.