Cnidaria possess tentacles armed with cnidocytes to capture prey, and exhibit a radial symmetry that can take the form of a sessile polyp or a floating medusa.

The gastrodermis lines a gastrovascular cavity for digestion; a single opening to this cavity functions as both mouth and anus. A gelatinous mesoglea is sandwiched between the epidermis and gastrodermis. A simple nerve net comprises its nervous system.