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.
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.
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.
Fig. 6.—Diagram of an eye of Leptoplana from the tentacle group. × 600. (After Lang.)
Reproductive Organs.—Leptoplana is hermaphrodite, and, as in most hermaphrodites, the reproductive organs are complicated. The male organs are the first to ripen, but this does not appear to prevent an overlapping of the periods of maturity of the male and female products, so that when the eggs are being laid, the male organs are, apparently, still in a functional state. The principal parts are seen in Fig. 5. The very numerous testes (te) are placed ventrally, and are connected with fine vasa efferentia (ve), which form a delicate network opening at various points into the two vasa deferentia (vd). These tubes, especially when distended with spermatozoa, may easily be seen (Fig. 2, vd) converging at the base of the penis, and connected posteriorly by a loop that runs behind the female genital pore (Fig. 5). The penis (pe) is pyriform and muscular, and is divided into two chambers, a large upper one for the spermatozoa, and a smaller lower one for the secretion of a special "prostate" gland. The apex of the penis is eversible and not merely protrusible, being turned inside out when evaginated. The ovaries (Fig. 5, ov) are numerous and somewhat spherical. They are dorsally placed, but when fully developed extend deeply wherever they can find room to do so, and they not only furnish the ova, but elaborate food-yolk in the ova, as there are no special yolk-glands. The slender oviducts (od) open at several points into the "uterus" (ut) (a misnomer, as no development takes place within it), which encircles the pharynx, and opens by a single duct into the vagina (va). Here the ova are probably fertilised, and one by one invested by the shell-gland (sg) with a secretion which hardens and forms a resistant shell. They are then laid in plate-like masses which are attached to stones or shells. The development is a direct one, and the young Leptoplana, which hatches in about three weeks, has the outline of a spherical triangle, and possesses most of the organs of the adult. After leading a floating life for a few weeks it probably attains maturity in about nine months.
Classification, Habits, and Structure of the Polycladida.
The Polyclads were so called by Lang on account of the numerous primary branches of their intestine. They are free-living, purely marine Platyhelminthes, possessing multiple ovaries, distinct male and female genital pores (Digonopora), but no yolk-glands. The eggs are small, and in many cases give rise to a distinct larval form, known as "Müller's larva" (Fig. 12). The Polyclads, with one exception,[[24]] fall into two sub-groups, Acotylea and Cotylea:—