Nervous System.[[131]]—The brain is composed of two ganglionic masses (Fig. 53, n.g) lying at the anterior end of the body, one on each side of the proboscis, and united by commissures passing round it (Fig. 55, d.c and v.c). Each ganglionic mass is often partially divided into a dorsal and ventral lobe (n.g.d and n.g.v). From the brain a pair of cords pass off backwards along the sides of the body (n.c); these cords, which have no ganglionic swellings, in some forms unite with one another above the anus. Anteriorly nerves are given off from the brain to the eyes and front part of the head (a.n). A nerve to the proboscis is given off from the commissure which unites the two halves of the brain dorsal to the proboscis (d.n).
Fig. 54.—Diagrammatic representation of the proboscis, (A) in the retracted condition, (B) in the everted condition. g.p, Glandular portion of the proboscis; m, muscle attaching the proboscis to its sheath; m.p, muscular portion of the proboscis; p.p in A, proboscis pore; p.p in B represents the position of the proboscis pore in the retracted condition of the proboscis; p.s, proboscis sheath.
In two out of the three groups into which the Nemertines are divided, the lateral nerve-cords are in connexion with a network or plexus of nerves lying between the muscular layers of the body-wall (Fig. 52, n.l), and in some forms constituting a comparatively thick layer. In these two groups there are no definite nerve branches except the anterior ones to the head. In the third group of Nemertines the lateral nerve-cords lie within the muscular layers of the body-wall, and in this case paired nerve branches are given off at definite intervals throughout the whole length of the body. These branches divide up among the organs to which they pass, and no nerve plexus is present.
The lateral cords vary in position in different cases. Sometimes they lie laterally, at others the cords tend to approximate to one another in the median dorsal or in the median ventral line, though in every case they remain distinctly separated.
Sense Organs.—Sense organs are usually present in the form of eyes arranged at the sides of the head (Fig. 51, e), sometimes as a single pair and sometimes in one or more groups on each side. The structure of the eyes varies from a simple pigment spot to an organ which receives a special nerve-supply from the brain, and possesses a refracting body answering to a lens, and behind this a pigment layer and a layer of rods. Some forms are devoid of all traces of eyes.
Fig. 55.—Diagram to show the relations of the nervous system, circulatory system, and proboscis sheath in the anterior end of the body in the Hoplonemertea, modified from M‘Intosh. a.n, Nerves to anterior part of body and eyes; d.c, dorsal commissure; d.n, median dorsal nerve; d.v, dorsal vascular trunk; l.v, lateral vascular trunk; n.c, lateral nerve-cord; n.g.d, dorsal lobe of nerve ganglion; n.g.v, ventral lobe of nerve ganglion; p.p, proboscis pore; p.s, proboscis sheath; v.c, ventral commissure; v.s, vascular ring or collar.
A pair of simple auditory capsules has been found in some of the Hoplonemertea, where they occur as small vesicles on the brain.
The whole surface of the body appears to be remarkably sensitive. In a few forms small tufts of tactile hairs are said to be present in the region of the head, while in others there are a few long hairs scattered sparsely among the cilia of the epidermis.