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.
Circulatory or Blood-Vascular System.—The circulatory system consists of three main longitudinal vessels, a median dorsal and a pair of lateral ones. These are connected together posteriorly by a transverse trunk, and also throughout the whole length of their course by branches, which are given off at regular intervals. Anteriorly the three longitudinal vessels either all unite and form a collar (Fig. 55, v.s) round the oesophagus, or they break up into a number of lacunar or open spaces in free communication with one another.
The blood is usually colourless, but in some cases the corpuscles are coloured red by haemoglobin.
Fig. 56.—Excretory system of Nemertines. A, Drepanophorus spectabilis Qtrf., part of one of the lateral vessels encircled by branches of the excretory organ, × 585; e, main canal of the excretory system: B, D. crassus Qtrf., a terminal branch of the excretory system, × 585; f, ciliated flame: C, Malacobdella grossa O. F. Müll., entire animal, slightly magnified, showing the excretory system (black) and the vascular system; e.a, excretory aperture; d.v, dorsal vessel; l.v, lateral vessel. (From Bürger.)
Excretory System.—Max Schultze[[133]] found in Tetrastemma obscurum, on the outer side of, but near to the lateral blood-vessels, a pair of canals. He observed ciliary movements in the canals, but could not discover flame cells. Further contributions to our knowledge of the excretory system were made by Semper,[[134]] von Kennel,[[135]] Hubrecht,[[136]] and Oudemans.[[137]] The latter states that the excretory system consists of a pair of canals situated laterally near the anterior end of the body. Each canal communicates with the exterior by one or more ducts having lateral regularly-arranged apertures. In some cases he was unable to make out any communication with the vascular system, but in others a direct communication, by means of open connexions with the lacunar blood spaces, is said to occur.
Silliman[[138]] in Tetrastemma aquarum dulcium describes the excretory vessels as ending in numerous capillary branches, at the blind terminations of which cilia are present. He states that there is no important difference between the excretory systems of Rhabdocoeles and Nemertines.