MEDUSA, in zoology, a genus of insects belonging to the order of mollusca. The body is gelatinous, roundish, and depressed; and the mouth is in the centre of the under part of the body. There are twelve species, all natives of the sea. The most remarkable is the simplex, or armless, with a plain circumference; four apertures beneath; no tenacula. These animals inhabit all our seas; are gregarious; often seen floating with the tide in vast numbers; feed on insects, small fish, &c. which they catch with their clasps or arms. Many species, on being handled, affect with a nettle-like burning, and excite a redness. The ancients, and some of the moderns, add that they have an aphrodisiac property, and in several languages they are called by an obscene name. They were known to the Greeks and Romans by the names of Πυρρα Θαλεσθ, and pulmo marinus, or sea-lungs. They attributed medicinal virtues to them. Dioscorides informs us, that, if rubbed fresh on the diseased part, they cured the gout in the feet, and kibed heels. Ælian says, that they were depilatory; and, if macerated in vinegar, would take away the beard. Their phosphoric quality is well known; nor was it overlooked by the ancients. Pliny observes, that if rubbed with a stick it will appear to burn, and the wood to shine all over. The same naturalist observes, that when they sink to the bottom of the sea, they portend a continuance of bad weather.