Cnidarians have bilateral symmetry, which is a characteristic that distinguishes cnidarians from other organisms. Cnidarians have complex levels of tissue organization and lack of structured internal organs. Additionally, adult cnidarians derive from two distinct embryonic germ layers, which are the ectoderm and endoderm.
Cnidarians vary in size, species and shape, but have radial symmetry that gives their bodies shape and facilitates important functions, such as digestion and reproduction. Cnidarians all have mouths, situated at their oral ends, which also contain mouths, teeth in tooth-bearing cnidarians, and sensory organs, such as antennae and noses. The opposite ends of these animals are called aboral, and contain end-digestive system components, such as intestines and anuses. Radial symmetry appears more distinctly in some cnidarians than others. However, it bears the same biological and physical traits in all cnidarians. Radial symmetry is characterized by a distinct plane that passes through the oral and aboral axis. This, in turn, divides species into two equal halves, generally a left and right side, which are mirror images of each other. Cnidarians also have similar body designs, which includes internal cavities with singular openings. Many cnidarians also have tentacles, and have unique organelles called nematocysts, which are found in cells called cnidoblasts. Cnidoblasts may occur in the epidermis and gastrodermis, but are frequently located on tentacles.