MEDUSA, in zoology, a genus of vermes, 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. 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 Θάλασσις, 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. Alian 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.
On Plate CCXCIV, are figured several specimens, viz. 1. The aurita, or aurited medusa; which appears, as floating on the water, to be a mere lifeless lump of jelly. It is of a whitish colour, with a cast of bluish grey, and is of an orbiculated figure, elevated into a convexity in the middle on the upper side, flat on the under, and furnished with a fringe of fine and somewhat rigid filaments round the edge, resembling white hairs: on the under surface there are four cavities near the centre, each of an arcuated figure, and surrounded with an opaque line, formed of about 24 parallel points or dots: from the very centre of the under side there arise four crooked appendages, which have each a row of hairy filaments on the exterior edge; and on the upper surface there is an appearance of fine vessels of a pale colour. This species is frequent, floating on the surface of the sea, or adhering to rocks about our own coasts; and when the sun shines on them, they have a very beautiful lucid appearance. It is called by some the sea nettle, it being one of those animals that when touched occasions a very disagreeable tingling in the hands.
2. The capillata, or capillated medusa, is a very singular and odd animal: it seems a mere lump of a whitish semi-pellucid jelly; and is as easily broken and destroyed by a touch as the common jellies brought to our tables: its shape is rounded, rising into a convexity in the middle, where it is therefore thickest, and whence it becomes gradually thinner to the sides: on the under side it is plain, and on this there is visible a rough, or as it were echinated circle, within which
there run eight pairs of rays from the centre toward the circumference; and from the centre there arise also a number of curled appendages, which are sometimes reddish, but more usually whitish, and a vast number of slender filaments: the edge or the circumference of the body is regularly divided into eight portions, and each of them is emarginated, so that on the whole verge there are 16 sinuses. This species is to be met with in vast abundance floating on the surface of the water about Sheppey island in Kent, and elsewhere on that coast: great quantities of it are destroyed by being thrown on shore with the waves, whence it has no power of getting off again; and in the open seas, many fish swim near the surface, and prey on them. This is the species called by many authors pulmo marinus, or the sea lungs. 3. The marupialis, or purple medusa, is semi-oval, with four tentacula on the edge. It inhabits the Mediterranean. 4. The waved medusa has the edges waved, with fangs on the projecting parts; four orifices beneath, between which rises a stem divided into eight large ragged tentacula.
These animals swim in large companies in search of food, with their tentacula in continual motion, with which they seize their prey, and convey it to their mouths: they vary in size, the largest being generally about eight inches in diameter. They vary likewise in the number of their tentacula; some having only two, others four, six, and some eight, but they rarely exceed that number. So powerful is their embrace, that whatever prey comes within their reach never escapes. They subsist on insects, small fish, &c.
Mr Banks, in his passage from Madeira to Rio de Janeiro, discovered a new species, which, when brought aboard by the casting net, had the appearance of metal violently heated, and emitted a white light. With these animals were taken small crabs of three different species, altogether new, each of which gave as much light as the glow-worm, though the creature was not so large by nine-tenths. These luminous animals are one of the causes of that appearance to the sea which has been mentioned by many navigators, and of which various reasons have been assigned. It appeared to emit flashes of light exactly resembling those of lightning, only not so considerable, but so frequent, that sometimes eight or ten were visible at the same moment.