NEREIS, (Encycl.) The head of this insect is roundish, flattened, with the rostrum extending to a point; tentacula (feelers) two, very short, subulate; eyes, two, prominent, on the sides of the head; body consisting of 23 segments, diminishing toward the tail; the side of every segment has a conical apex cut short at the vertex, and denticulated; from which springs a small pencil of hairs, under each pencil a filament resembling a foot, subulate, flexible, unarmed. Colour, glassy, pellucid. Length, two lines. Habitation, sea-plants, as the fuci and algæ, which sometimes it leaves, and wanders on the surface of the sea.
This insect is frequent in all seasons of the year, but particularly in summer, and on the approach of a tempest, when it seems through inquietude to give a much greater quantity of light; its appearance also is very brilliant when the wind is in the east and south-east points, and in winter-nights preceded by a warm day. If water containing these animacules be kept warm, they retain their light two whole days after they are dead; but in cold weather lose it in eight hours: motion and warmth, which increase their vivacity and strength, increase their light also.