In _Nereis_ there is in each segment, in addition to the pair of nephridia, a pair of "dorsal ciliated organs" (Goodrich) (_cil.org_ in Fig. 124). Each appears as a wide-mouthed funnel, greatly folded, and without any permanent outlet. But it is possible that these organs function as genital ducts, and that the external aperture will make its appearance temporarily at the period of maturity. This "dorsal ciliated organ" has not been met with in allied genera—such as _Eunice_, _Nephthys_, _Polynoë_, _Glycera_—where the nephridium is a wide tube, and serves as a genital duct. [Illustration: FIG. 129.—Nephridium of _Arenicola_. (From Benham.) × 4. _d_, Dorsal lip of funnel; _v_, ventral lip of funnel; _b_, blood-vessel (all the black lines are blood-vessels); _m_, dilated bladder; _x_, part cut away from body-wall where the nephridium is passing to the exterior.] The NERVOUS SYSTEM, as in all Chaetopods, consists of a dorsal cerebral ganglion or "brain" (Fig. 127, _br_), connected by circum-buccal commissures with the anterior end of a ventral chain of ganglia. The brain occupies the prostomium,[309] and from it nerves pass away to the prostomial tentacles and palps. The circum-buccal commissures spring from the outer corner of the brain, and from each arises a nerve to the first pair of peristomial cirri. The first ventral ganglion lies in the third segment, and represents at least two ganglion-pairs fused together, for from it arise (1) a pair of nerves to the second pair of peristomial cirri and (2) a {255}pair to the first parapodium. In the remainder of the body there is a ganglion in each segment, whence nerves pass outwards to the parapodium and muscles of the segment (Fig. 124). In _Nereis_ the apparently single ganglion in each segment really consists of two halves, and the apparently single cord which traverses the whole length of the body consists of two closely apposed cords. In some worms, such as Serpulidae, the two cords are more or less widely separated, and the two ganglia of each segment are thus distinct, and connected by a transverse commissure. In _Nereis_, as well as in many other Polychaeta, the nerve-cords lie within the body-wall, but in other cases they lie in the epidermis, as they do in Archiannelida. The visceral nervous system, supplying the muscles of the pharynx, is frequently highly developed. In _Nereis_ it arises on each side by two roots, one from the brain, the second from the circum-buccal commissure. [Illustration: FIG. 130.—Eye of _Nereis_. (After Andrews.) × 150. A, Section through the entire eye; _c_, cuticle; _e_, epidermis; _l_, lens; _h_, rods; _r_, retina; _n_, optic nerve: B, isolated retinal element; _c_, cell; _p_, pigment; _h_, rod: _f_, nerve-fibre.]