The excretory system is represented by a pair of nephridia in each segment, with the exception of a few anteriorly and a few posteriorly. The nephridium of Nereis differs from that of most other Polychaetes hitherto examined carefully, and rather resembles that of the Oligochaetous Enchytraeids. It consists of a compact gland-like organ, containing a much coiled tube, ciliated for the greater part of its length, but deprived of cilia in its last coils; this latter part—or duct—leaves the "gland" and pierces the body-wall, opening to the exterior at the base of the parapodium. The ciliated canal passes forwards into the next segment, where it opens by a funnel into the coelom. The lip of the funnel is extremely curious, for the cells constituting it are drawn out into very long, delicate processes covered with cilia.[[307]]
Fig. 128.—Nephridium of Nereis. (From Goodrich.) f, Funnel; n, neck, which passes through a septum; t, coiled tubule; c, connective tissue; d, duct.
In most Polychaetes the nephridium is a wide, sac-like tube as in Arenicola[[308]] (Fig. 129). Its walls are covered by a dense network of blood-vessels, and it not only acts as an excretory organ, but also as a genital duct (see p. [273]).
Excretion, in the strict sense of the word, is carried out by the cells forming the wall of the tube; they remove waste materials from the blood distributed over the surface of the organ. But, in addition, there is a removal from the coelom, by means of the funnel, of any dead or dying coelomic corpuscles which in their turn have eaten up or otherwise destroyed foreign bodies (such as Bacteria, etc.) that may have entered the animal.
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.
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 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.