The alimentary canal is lined throughout by a ciliated epithelium. The oesophagus has, in addition to this layer, an outer thick coat of large granular cells, which probably have a glandular function. PROBOSCIS.—The most characteristic organ of the Nemertines is the proboscis (Figs. 50, 53, 54). For many years its disposition and function were misunderstood, and it was supposed to be a portion of the digestive system. The proboscis, which lies dorsal to the alimentary canal, opens at the extreme anterior end of the body by a small pore (Figs. 51, 53, 58). When retracted it is sometimes considerably folded, and lies in a long pouch or sheath. To the walls of this sheath it is attached round its anterior {104}end; and strong muscles unite its posterior extremity to the sheath a short distance from the posterior end of the latter. The proboscis seems to be exclusively a tactile and protective and defensive organ, for which functions it is eminently fitted by the great ease and rapidity with which it is everted or thrust out from the body. It consists of two distinct regions (Fig. 54, _g.p_ and _m.p_). In the retracted state the anterior part is a hollow tube with very thick muscular walls made up of several layers. At the base of this part in many of the Nemertines there is situated a sharp-pointed spine projecting forward into the lumen, and several smaller stylets situated in a pair of vesicles close to the base of the central spine. The position of the spines in the everted proboscis is shown in Fig. 57. The posterior part of the proboscis is also a tube, but instead of being muscular, its walls are glandular. This posterior glandular part is never everted. [Illustration: FIG. 53.—Diagrammatic drawing of a Nemertine from the dorsal surface to show the position of some of the principal organs. _a_, Anus; _c.s_, cephalic slit; _g_, generative organs; _int_, intestine with its lateral diverticula; _m_, mouth; _n.c_, lateral nerve-cord; _n.g_, nerve ganglion; _oes_, oesophagus; _p_, proboscis; _p.p_, proboscis pore; _p.s_, proboscis sheath.] The eversion is effected by a turning inside out of the anterior part of the proboscis (Fig. 54). The process whereby the proboscis is retracted has been very aptly compared to the effect which would be produced by the inversion of the finger of a glove, accomplished by pulling a string attached to its tip on the inside, the anterior muscular part being comparable to the finger and the glandular part to the string. It is thus obvious that in the everted condition the stylet will form the anterior tip of the {105}proboscis, and will there be in a position for offence or defence (Fig. 57, _s_).