A TYPICAL BODY SEGMENT carries on each side a muscular lobed outgrowth, bearing bundles of bristles or "chaetae," and filamentous sensory organs known as "cirri." To this lateral locomotor organ Huxley gave the name "parapodium" (Fig. 124). Each parapodium or foot consists of a basal portion, supporting a dorsal and a ventral process, the "notopodium" (_ntp_) and "neuropodium" (_nrp_) respectively, each of which is bilobed. The lobes are very vascular and glandular, and probably serve as respiratory organs or "gills." [Illustration: FIG. 122.—_Nereis pelagica_ L., natural size. (From Ehlers.)] [Illustration: FIG. 123.—Chaetae of _Nereis_; enlarged. A, from neuropodium; B, from notopodium of _N. diversicolor_; C, swimming chaeta of Heteronereid stage of _N. dumerilii_.] The chaetae, or bristles, of each bundle project from the mouth of a great sac, the lips of which are particularly prominent in _Nereis_. Each chaeta arises from a single cell situated at the bottom of the sac. The chaetae of _Nereis_, as of many other Polychaetes, are of a kind usually termed compound or "jointed," each being composed of a long stalk and a small "appendix" articulated in a cup at its {247}extremity (Fig. 123). The shape of the cup varies; it is in some cases of equal height all round, or it is higher on one side than on the other. Further, the appendix may be short and curved, or more elongate and spear-like; it is generally notched or finely toothed on one side. [Illustration: FIG. 124.—_Nereis._ Somewhat diagrammatic transverse section through the body. On the left the chief constituents of the vascular system are represented; on the right side the chaetae and their muscles, as well as the distribution of the lateral nerve, etc., are shown. _ac_, aciculum; _bv_, network of blood-vessels; _ch_, chaetae (only a few are shown) in two bundles; _ch.l_, lips of the chaetigerous sac; _cil.org_, dorsal ciliated organ; _cir_, circular muscular coat; _coe_, coelom; _d.cir_, dorsal cirrus; _d.long_, dorsal bundle of longitudinal muscles; _d.ve_, dorsal blood trunk; _ep_, epidermis; _INT_, intestine; _int.cap_, blood capillaries in its wall; _m.ch_, muscles which move the chaetae; _N.c_, ventral nerve cord; _ner_, lateral sensory nerve, dividing into a ventral branch entering the ventral cirrus, and a dorsal branch (_n.cir_) for the dorsal cirrus; _neph_, nephridium, seen through the oblique muscle through which its funnel passes; _nrp_, neuropodium; _nrp.lig_, neuropodial lobe or ligule; _ntp_, notopodium; _ntp.lig_, notopodial ligule; _obl_, oblique transverse muscle (muscle of the parapodium); _pv_, peripheral blood-vessel; _v.cir_, ventral cirrus; _v.long_, ventral bundle of longitudinal muscles; _v.ve_, ventral blood trunk.]