[Illustration: 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 {107}are a few long hairs scattered sparsely among the cilia of the epidermis. _Frontal Organ._—In many Nemertines there is present at the anterior tip of the head a disc-shaped group of cells bearing long hairs or bristles. On this disc open the secreting ducts of a number of gland cells lying in the head. It seems possible that this frontal organ may function as an organ of taste. _Side Organs._—In the Carinellidae there is a pair of circular epithelial patches lying one on each side of the body in the region of the excretory pore. The cells composing them are richly ciliated and provided with a plentiful nerve-supply. The function of these epithelial patches is not known, but it has been suggested that they may be auditory organs. _Cephalic Slits and Cerebral Organs._—In most Nemertines there is a peculiar pair of organs (Figs. 50, 53, _c.s_), situated in the head and in close connexion with the brain. The function of these organs is not known. Hubrecht has suggested that they may be respiratory, while Bürger[132] conjectures that they may be organs which are used for discriminating the condition of the surrounding medium. In an external examination of the head, the cephalic slits may usually be seen as a pair of lateral furrows or pits. Their form and direction vary considerably; they may take the form of shallow circular depressions, or they may lie longitudinally and be slit-like in shape (Fig. 50), or the slit may lie at right angles to the long axis of the body and be beset with short transverse furrows. In some forms these slits are merely superficial depressions, but in others they are continued into ciliated ducts, which pass inwards and penetrate into special lobes, consisting of glandular tissue and ganglion cells, in close connexion with the brain. These lobes are called the cerebral organs. In many forms the nervous system is charged with haemoglobin, which gives to it a bright red colour.