NEREIS, in zoology, a genus of animals belonging to the order of vermes mollusca. The body is oblong, linear, and fitted for creeping; it is furnished with lateral pencilled tentacula. There are 11 species; of which the most remarkable are the five following: 1. The Noctiluca, or noctilucon nereis, which inhabits almost every sea, and is one of the causes of the luminousness of the water. These creatures shine like glow-worms, but with a brighter splendour, so as at night to make the element appear as if on fire all around. Their bodies are so minute as to elude examination by the naked eye.
It is sometimes called nereis phosphorans; and is thus described by Grifelin: The head is roundish and flat, and the mouth acuminate. The two horns or feelers are short and subulated. The eyes are prominent, and placed on each side the head. The body is composed of about 23 segments or joints, which are much less nearer the tail than at the head. These segments on both sides the animal all end in a short conical apex, out of which proceeds a little bundle of hairs: from under these bundles the feet grow in the form of small flexible subulated filaments destitute of any thing like claws. It is scarcely two lines long, and is quite pellucid, and its colour is that of water green. They are found upon all kinds of marine plants; but they often leave them and are found upon the surface of the water: they are frequent at all seasons, but especially in summer before stormy weather, when they are more agitated and more luminous. Their numbers, and wonderful agility, added to their pellucid and shining quality, do not a little contribute to their illuminating the sea, for myriads of those animalcule may be contained in the portion of a small cup of sea-water. Innumerable quantities of them lodge in the cavities of the scales of fishes, and to them probably do the fishes owe their noctilucon quality. "I have observed with great attention (says Barbut), a fish just caught out of the sea, whose body was almost covered with them; and have examined them in the dark: they twilt and
curl themselves with amazing agility, but soon retire out of our contracted sight; probably their glittering numbers dazzling the eye, and their extreme minuteness eluding our researches. It is to be observed, that when the unctuous moisture which covers the scales of fishes is exhausted by the air, these animals are not to be seen; nor are the fishes then noctilucon, that matter being perhaps their nourishment when living, as they themselves afford food to many marine animals. They do not shine in the day-time, because the solar rays are too powerful for their light; however aggregate or immense their number." Their appearance is particularly 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 animalcules be kept warm, they retain their light two whole days after they are dead; but in cold water lose it in eight hours: motion and warmth, which increase their vivacity and strength, increase their light also.
2. Nereis lacustris, or bog nereis. The body of the size of a hog's short bristle, transparent, as it were articulated, and on either side at every articulation provided with a short setaceous foot; interiorly it seems to consist in a manner of oval shaped articulations, and a back formed by two lines bent backwards. It inhabits marshes abounding in clay, where it remains under ground, pushing out its other extremity by reason of its continual motion. When taken out it twists itself up. Is frequent in Sweden.
3. Nereis cirrosa, or waving nereis. The body is red, lumbriciform, with 65 notches, furnished on both sides with two rows of bristles. At each side of the head ten filaments, at the sides of the mouth many, twice as long as the former. It dwells in Norway, on rocks at the bottom of the sea. It vomits a red liquor with which it tinges the water.
4. Nereis carulea, or blue nereis. It inhabits the ocean; where it destroys the sponges and teredines.
5. Nereis gigantea, or giant nereis. This is a peculiar species of those large worms that make their way into decayed piles driven down into the sea, which they bore through and feed upon, whence they are called sea worms or nereis. From head to tail they are beset on either side with small tufts terminating in three points; which are like the fine hair pencils used by painters, and composed of shining bristles of various colours. The upper part of the body in this worm is all over covered with small hairs. The rings of which it is formed are closely pressed together, and yield to the touch. The three rows of small tufts we have been describing, serve this nereis instead of feet, which it uses to go forwards as fishes do their fins.