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Jellyfish are members of the phylum Cnidaria. Members of this structurally simple marine group possess one of two body forms. Sea anemones, sea whips, corals and hydroids are polyps growing attached to rocks or other hard surfaces of the sea. Jellyfish and the Portuguese man-of- war are free-swimming medusae. Both body forms display radial symmetry with body parts radiating from a central axis. This symmetry allows jellyfish to respond to food or danger from any direction. 

Instead of a brain, "true" jellyfish possess an elementary nervous system, or nerve net, which consists of receptors capable of detecting light, odor and other stimuli and coordinating appropriate responses.

Jellyfish are composed of an outer layer (epidermis) which covers the external body surface and an inner layer (gastrodermis) which lines the gut. Between the epidermis and gastrodermis is a layer of thick elastic jellylike substance called mesoglea ("middle jelly"). Jellyfish have a simple digestivecavity (coelenteron) which acts as a gullet, stomach and intestine with one opening for the mouth and anus. Four to eight oral arms are located near the mouth and are used to transport food that has been captured by the tentacles.

Jellyfish occur in a wide variety of sizes, shapes and colors. Most are semi-transparent or glassy and bell-shaped, measuring less than an inch to over a foot across the bell, although some may reach 7 feet. The tentacles of some jellyfish can reach lengths greater than 100 feet. Regardless of their size or shape, most jellyfish are very fragile, often containing less than 5% solid organic matter.