Nerilla is an Archiannelid, with a prostomium, a pygidium, and nine trunk segments bearing parapodia. There are three cirri on the prostomium, two on the pygidium, and one on each parapodium except the last. The cirri are slender, hollow, jointed processes, with sensory hairs. A ventral ciliated groove extends from mouth to anus ; ciliated rings occur on the prostomium and on the trunk segments, and there is a ciliated patch on each side of each segment. The first parapodium bears one bundle, and the other parapodia two bundles of simple cliEetas. The cirrus is placed between the bundles. The central nervous system, throughout closely connected with the epidermis, consists of a brain and two ventral, nerve-cords without ganglionic swellings. The prostomium carries four eyes and two nuchal organs. The alimentary canal consists of an oesophagus, stomach, intestine and short rectum, all ciliated, and a ventral muscular pharynx. Unicellular glands open into the buccal cavity, and gland-cells are greatly developed on either side of the stomach. The cœlomic is well developed; but some of the septa are very incomplete. Cœlomic corpuscles are very rare; a delicate network of cœlenchyme extends throughout the body-cavity. A simple vascular system is present. The nephridia have open funnels, a slightly coiled lumen, and vacnoles containing one refringent concretion. Nephridia are present in segments 2, 5 and 9 of the male, and in segments 2, 5, (5 and 8 of the female. The male has three genital segments, 5, 6, and 7. In the first only do the testes produce spermatozoa ; in the second and third the testes appear to be degenerate and to produce granular cells, with small granules in segment 6, and large granules in segment 7. Three pairs of similar ducts correspond to these three segments. The six sperm-ducts open into a common median ventral genital atrium, which leads to a median pore with a small copulatory process on each side. Special epidermal gland-cells open into the genital atrium. Two ovaries are present in the sixth segment of the female. Each ovary becomes enclosed in an ovisac, lined by nutritive epithelium ; the ovisacs are asymmetrically developed. Full-grown ova escape from the sacs into the cœlom of the sixth segment, and are retained for a long time in the parent, passing forwards or more often backwards along each side of the gut. Here they undergo the preliminary stages of maturation. It is possible, but not yet certain, that fertilisation is internal. A pocket, containing deeply staining cells, is found on the inner side of encli ovisac. Two oviducts arc present, leading from the sixth segment to paired genital pores on the seventh segment. Epidermal glands converge towards the pores. The eggs are laid in transparent capsules, development is direct, and the young emerge as small worms with only one median tentacle and about five segments.
Nerilla combines primitive with specialised characters. The parapodia, chfette, ccelom, and number of the gonads and their ducts have become progressively reduced in the Archiiinnelids. They are probably modified forms descended from Chætopod ancestors. The genital ducts appear to be either cœlomostomes, or cœlomostomes combined with nephridia (nephronixia). Nerilla preserves many Chætopod characters, but has affinities with several genera, more especially perhaps with Histriobdella and Dinophilus.