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.

Includes Hydras, hydroids, hydromedusae, chondrophorans, siphonophorans, and hydrocorallines. These animals are characterized by having a non-cellular mesoglea, a gastrodermis lacking nematocysts, and gonads in the epidermis. They may have either the poly or medusa form, and a number have both.