FAMILY ~NEREIDÆ~ GENUS ^Nereis^ [Illustration: Head of /Nereis pelagica/.] Among the most common of the polychæte worms is the genus ^Nereis^, various species of which are found in all parts of the world. They are abundant under stones, among seaweeds, and living in burrows between tide-marks. They are active, fierce, and voracious. Some attain a length of eighteen or more inches. They are commonly known as "clam-worms," and are used by fishermen for bait. /Nereis/ has a distinct head, consisting of two parts. One, the /prostomium/, bears on its upper side four eyes and a pair of short, conical tentacles, and on the lateral sides a second pair of processes called /palps/. The palps are more conspicuous than the tentacles, and consist of two parts, a large base and a small terminal point, the latter capable of being withdrawn. The palps are sense-organs and perhaps test the food. The second part of the head, or /peristomium/, bears on the sides four pairs of long tentacles which are used as feelers. The mouth is on the ventral side, and through it /Nereis/ throws out its pharynx, which contains a horny, notched jaw. It seizes its prey with this jaw, which, with the pharynx, is then withdrawn and tears the food apart, acting like a gizzard. The body of /Nereis/ is rounded above and nearly flat below. Each of the segments has a pair of parapodia, bearing a bundle of bristles and cirri. The last segment of the body is elongated, cylindrical, and without parapodia, but has on the end a pair of long cirri, which give the appearance of a divided tail. Each segment of the body, except the head- and tail-segments, contains a pair of excretory tubes (/nephridia/), ganglia of nerves, and a portion of the intestine and of the vascular system. There are circular muscles by which the worm can diminish its diameter, longitudinal [pg177] muscles in four bands, and muscles to move the parapodia. Breathing is carried on over the whole surface of the body, and especially in parts of the lobes of the parapodia called gills. Its sense-organs are eyes, palps, tentacles, and cirri. ^N. virens.^ This species is found from New York northward in muddy and shelly sand, and under rocks between tide-marks, living in burrows, which it lines with a mucous secretion. It is very active, and voracious, feeding on other worms, /Crustacea/, etc., which it captures with its horny, protruded jaw. At night it leaves its burrow and swims freely about like an eel, frequently falling a prey to fishes. In color it is dull bluish-green, with some iridescence. The gills, which are leaf-like appendages on the parapodia, are green on the anterior end of the body and become bright red farther back. This species, and /N. branti/ of Alaska, are the giants of polychæte worms, often measuring eighteen inches or more in length. [Illustration: /Nereis pelagica./ 1, male; 2, female.]