{14}[Illustration: FIG. 5.—Diagrammatic view of the structure of _Leptoplana tremellaris_ as a type of the Polycladida. The body is cut across the middle to show the relative position of organs in transverse section. In the posterior half the alimentary canal has been bisected and removed from the left side, to exhibit the deeply placed nervous sheath (_nn_) and the male reproductive organs. _br_, Brain; _dp_, "diaphragm"; _e_, cerebral group of eyes; _et_, tentacular eye-group; _gr_, marginal groove; _gm_, true mouth; _lg_, lateral gut-branch; _ln_, longitudinal nerve stem; _m_, external mouth; _mg_, _mg'_, main-gut, whole, and bisected; _n_, sensory nerve supplying the eyes; _nn_, nervous network lying on the ventral musculature; _n.s_, lateral nerve; _od_, oviduct; _ov_, ovary; _pe_, penis (in section); _ph_, pharynx; _pr_, prostate or "granule gland"; _sc_, sucker; _sg_, shell-gland; _te_, testes; _up_, anterior unpaired gut-branch; _ut_, uterus; _va_, vagina (in section); _vd_, vas deferens; _ve_, vasa efferentia; ♂, male genital pore; ♀, female pore.] {15}SENSE ORGANS.—_Leptoplana_ possesses eyes, stiff tactile, marginal cilia, and possibly a sense organ in the "marginal groove." The eyes, which are easily seen as collections of black dots lying at the sides of the brain, may be divided into two paired groups: (1) cerebral eyes (Fig. 5, _e_), and (2) tentacle eyes (_et_), which indicate the position of a pair of tentacles in allied forms (Fig. 8, A, _t_ and B). Each ocellus consists of a capsule placed at right angles to the surface of the body in the parenchyma, below the dorsal muscles, and with its convex face outwards. It is a single cell in which pigment granules have accumulated. The light, however, can only reach the refractive rods, which lie within it, obliquely at their outer ends. These rods are in connexion with the retinal cells, and thus communicate by the optic nerve with the brain. The cerebral eyes are really paired, and are directed some upwards, some sideways, some downwards. The "marginal groove" is a shallow depression of the epidermis (Fig. 5, _gr_) lined by cilia, and containing the ducts of very numerous gland-cells. It runs almost parallel to the anterior margin of the body, a short distance from it, but we have no observations on its functions. [Illustration: FIG. 6.—Diagram of an eye of _Leptoplana_ from the tentacle group. × 600. (After Lang.)]