Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. They have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction-pulsations of the bell-like body. The jellyfish body consists of over 95% water; most of their umbrella mass is a gelatinous material (the jelly) called mesoglea, which is surrounded by two layers of protective skin. The top layer is called the epidermis, and the inner layer is referred to as the gastrodermis, which lines the gut.

Jellyfish are found in every ocean, from the surface to the deep sea. A few jellyfish species inhabit freshwater. Large, often colourful, jellyfish are common in coastal zones worldwide. Jellyfish have roamed the seas for at least 500 million years, making them the oldest multi-organ animal.