Nereids, the name of the sea centipedes of the dorsibranchiate order of annelids, of which the genus nereis (Cuv.) is one of the best known forms. The class and ordinal characters are given under Annelida. In the true nereids the tentacles are of even number; there is a single pair of maxillae in the proboscis; the branchiae are small plates in which a network of blood vessels is disposed; each of the numerous feet has two tubercles, two bundles of bristles, and an under and an upper cirrus; the body is like that of a centipede, with many joints. They are active, crawling and swimming with facility; some live in crevices in rocks near the shore, in sponges, corals, deserted shells under stones, or wherever they can find a shelter; others burrow in mud or sand, occasionally making tubes or sheaths, from which they protrude the anterior part of the body, and into which they retreat quickly when alarmed; some of the tropical species are very large, and vividly phosphorescent at night. Their food consists of living and dead animal substances, such as marine worms and soft-bodied invertebrates.

The species are numerous, and are found on almost all coasts. - The sea mice (aphrodita, Linn.) belong to the same order; the body is broad and flat, with two longitudinal rows of wide membranous scales covering the back and concealing the branchiae. The common sea mouse (A. aeuleata, Linn.) is 6 or 7 in. long and 2 broad, the back covered with a flax-like substance in which are mixed spines and softer bristles, the latter shining with all the tints of the rainbow, and rivalling in beauty the lustre of the humming bird or the sparkling of the most brilliant gems.

Common Sea Mouse (Aphrodita aeuleata).

Common Sea Mouse (Aphrodita aeuleata).