The lateral cords are placed towards the dorsal aspect of the body in Langia (Schizonemertea), towards its ventral aspect in Drepanophorus (Hoplonemertea). The supra-anal commissure is found in Hoplonemertea. The ganglia and lateral nerves of Schizonemertea are deeply tinged with haemoglobin. Dorsal and ventral nerves which branch dichotomously originate metamerically from the lateral nerves in Hoplonemertea. But in other Nemertea the lateral nerves are contained in a nervous sheath or plexus, which envelopes the body and in Carinella extends beyond the ganglia to the tip of the head, a region where definite nerves are found in Schizonemertea. The nervous system lies in Carinella immediately outside the muscular body-walls, in Polia Vaiencinia, and Schizonemertea internal to the outer longitudinal muscular coat, and in Hoplonemertea within the body-walls.
The organs of special sense are small tufts of tactile hairs, sometimes seen at the apex of the head; long stiff isolated hairs on the surface of the body, observed only in young specimens; eyes sometimes absent, e. g. in mud-dwelling Schizonemertea, either numerous or restricted to four as in Tetrastemma, in structure either mere pigment specks or provided with a lens, a cellular vitreous body, retinal rods and a pigment sheath as in Drepanophorus and Amphiporus; and otocysts stated to occur in some Hoploiiemertea. The two ciliated cephalic grooves situated on the head are probably not sensory organs. These structures are absent in Cephalothrix and Geonemertes chalicophora. In the Palaeonemertean Carinella annulata they form two simple shallow furrows at the level of the cerebral ganglia.
1In Carinella and Cephalothrix the ganglion layer is confined to one, the outer side of the fibrous core.
A ciliated canal leads inwards from each of them and enters the cerebral ganglia in C. inexpectata, or their posterior or third lobes in Schizo- and Hoplonemertea1. In the Schizonemertea the groove is longitudinal and deep; in most Hoplonemertea transverse and often complicated by the addition of short longitudinal grooves present also in the Palaeonemertean Polia2. The ciliated canal of Hoplonemertea is bifurcate.
The aperture by which the proboscis is extruded is anterior and terminal as a rule, and rarely lies within the mouth (Akrostomum, Geonemertespalaensis, Malacobdella, Monopord). It leads into a narrow canal. The proboscis proper commences just anteriorly to the ganglia. It is tubular, and is contained within a proboscis-sheath. The walls of the two structures are continuous in front, but elsewhere they are separated by a cavity filled with a liquid in which are suspended fusiform corpuscles, tinged with haemoglobin in Cerebratulus urticans. This cavity is completely closed. It is furnished in Drepanophorus with paired lateral diverticula commencing at the level of the nerve ganglia. The sheath lies dorsally to the digestive tract and usually reaches the posterior extremity of the body. Sometimes, e. g. Carinellay Nemertes carcinophila, it is extremely short. It has muscular walls, usually an outer circular and an inner longitudinal layer, whilst its cavity is lined by an epithelium. The proboscis also has muscular walls, an external and internal circular with an intervening longitudinal layer in Palaeo- and Schizo-nemertea; an outer and inner longitudinal layer with an internal and circular layer in Hoplonemertea. The longitudinal muscles are continued beyond the posterior end of the proboscis as the retractor muscle, which is affixed to the sheath somewhere near its middle region.
Numerous longitudinal nerves occur in the proboscis, as in Amplii-porus and Drepanophorus (Hoplonemertea), or a nervous sheath, as in Cere-bratulus (Schizonemertea). The proboscis appears to be everted by the contraction of the sheath upon its contained liquid, but its eversion is only partial. The epithelium lining it is derived from the ectoderm. The surface of the eversible region is often covered with adhesive glandular papillae; its epithelium sometimes contains nematocysts as in Cari7iella, Cephalothrixy Borlasia, and especially Cerebratulus. The limit of the eversible region in Hoplonemertea is marked by an internal constriction due to a great development of muscular tissue (the muscular bulb), surrounding a cavity or reservoir, which communicates with both the eversible and non-eversible sections of the organ. A central, partially calcareous stylet, pointed, or in Drepanophorus serrated, is implanted close to the outer opening of the reservoir; and on either side of it a narrow duct leads to a single small sac, rarely more, containing two or three calcareous stylets pointed at one end, at the other provided with a head like a nail's. Both central and lateral stylets can be thrown off and formed anew.
The muscular bulb contains glands; its reservoir is filled with liquid which is perhaps poisonous. The non-eversible section of the proboscis has glandular walls and is filled with liquid. The function of the organ is doubtful. It may be in part tactile, in part offensive or defensive1.
1Hence they have been supposed to possess a respiratory function in relation with the brain, especially in Schizonemertea, where haemoglobin exists in the nervous substance.
2The Palaeonemertean Valencinia has ciliated canals, but no grooves.
The mouth is ventral, in front of the ganglia in Hoplonemertea, behind them in other Nemertea. It leads into a straight oesophagus with longitudinal muscular fibres in its walls; an intestine which, except in the adult Carinella, Cephalothrix, and Carinoma, is provided with short lateral and usually opposite caeca. The latter in Pelagonemertes number only thirteen pairs, are long and branched at their ends. Both oesophagus and intestine are ciliated and contain glands in their walls. The anus is terminal. There is a system of coelomic blood spaces communicating anteriorly in the head and posteriorly by a supra-anal commissure. The spaces in the head and oesophageal region of Palaeo- and Schizo-nemertea are lacunar, i. e. large and somewhat irregular in outline, whereas posteriorly they become vessels, i. e. smaller and either circular or oval in section, as they are throughout the whole system in Hoplonemertea. They are lined throughout by an epithelium, and the vessels generally possess a muscular wall of circular and sometimes also longitudinal fibres.