DIGESTIVE SYSTEM.—The general arrangement of this system may be seen in Figs. 2, 5, and 7; and may be compared, especially when the pharynx is protruded, as in Fig. 2, with the gastral system of a Medusa. The "mouth" (there is no anus) is placed almost in the centre of the ventral surface. It leads (Fig. 7, B, _phs_) into a chamber (the peripharyngeal space) divided into an upper and a lower division by the insertion of a muscular collar-fold (the pharynx, _ph_), which may be protruded, its free lips {13}advancing, through the mouth (Fig. 2), and is then capable of enclosing by its mobile frilled margin, prey as large as _Leptoplana_ itself. The upper division of the chamber communicates by a hole in its roof[20] (the true mouth, Figs. 5 and 7, _g.m_) with the cavity of the main-gut or stomach (_m.g_), which runs almost the length of the body in the middle line, forwards over the brain (Fig. 5, _up_). Seven pairs of lateral gut-branches convey the digested food to the various organs, not directly however, but only after the food mixed with sea-water has been repeatedly driven by peristalsis first towards the blind end of the gut-branches and then back towards the stomach. Respiration is probably largely effected by this means. The epithelium of the intestine (Fig. 4, _l.g_) of a starving specimen is composed of separate flagellated cells frequently containing "yellow cells."[21] After a meal, however, the cell outlines are invisible. Gregarines, encysted Cercariae, and Orthonectida[22] occur parasitically in the gut-branches. An EXCRETORY SYSTEM of "flame-cells" and fine vessels has hitherto been seen only by Schultze[23] in this species, which will not, however, resist intact the compression necessary to enable the details to be determined. They are probably similar to those of _Thysanozoon_ described on p. 25. NERVOUS SYSTEM.—The brain, which is enclosed in a tough capsule (Fig. 5, _br_), is placed in front of the pharynx, but some distance behind the anterior margin of the body. It is of an oval shape, subdivided superficially into right and left halves by a shallow depression, and is provided in front with a pair of granular-looking appendages, composed of ganglion-cells from which numerous sensory nerves arise, supplying the eyes and anterior region. Posteriorly the brain gives rise to a chiefly motor, nervous sheath (Fig. 5, _nn_), which invests the body just within the musculature. This sheath is thickened along two ventral lines (Fig. 5, _ln_) and two lateral lines (_n.s_), but is very slightly developed on the dorsal surface. Ganglion-cells occur on the course of the nerves, and are particularly large at the point of origin of the great motor nerves.