172 PLANARIANS external aperture, (2) the secondary branches of these, and (3) the excretory cells with the fine tubules leading from them. .Rarely is there but a single main excretory trunk present opening at the hinder end of the body (Stcno- stonia); generally there are a pair of such trunks which open in common at the hinder end of the body, or separately (most Rhabdocoela), or by the mouth (fig. 3). In the Tridadid .i there are two or four lateral trunks present which open by a number of pores arranged in pairs upon the dorsal surface of the body ; the same appears to be the case in the Polycladida. The main trunks of the excretory sys tem are generally much twisted in their course, and anastomose with each other ; they receive the fine tubules either directly or, as in the Rliabdocri la, there is a network of secondary tubules interposed. The excretory cells are pear-shaped ; they are branched and furnished with a nucleus and a large vacuole which is directly continuous with the lumen of the tubule ; from the boundary wall of the vacuole springs a single fiagellum, which depends into the lumen of the tubule and is capable of active movement. Lang discovered in a marine form of the Tridadida (G unda) similar vacuo- FIG. s.-Mam minks of the ex- . . ii-i i ,, 1. cretory system of Mesottoma lated cells With a Single liagelluni ehrenbergii, 0. Sch. Open irnomr the emthelial cells of the ou to the exterior through the mouth, ph, pharynx. intestine, and came to the conclu sion that the excretory cells were on that account derived from the epithelium of the intestine. The movements of the excretory fluid towards the external pore are directed by this flagellum as well as by cilia developed upon the walls of the fine tubules ; the motion of all these cilia is such as to drive the contents of the tubules towards the excretory pore. The main trunks of the excretory system are either sparsely (Tridadida according to Jijima) or com pletely (Polycladida according to Lang) lined with cilia. Nervous System. The central organ of the nervous system, the brain (en), is a double ganglion at the anterior end of the body, and has been noticed in all the known forms with the exception of the Acoela. It is situated in front of or above the pharynx ; in those species in which a process of the intestine extends beyond the region of the brain (cf. figs. 7 and 8 viewed from the ventral surface) it is placed below this. In such cases there is sometimes a com missure encircling the prolongations of the intestine. Each of the two ganglia gives off a strong longitudinal nerve cord (figs. 5-8, In) from which arise branches going to the various organs of the body. The structure of the nervous system is somewhat different in the Rhabdocoela, Trida dida, and Polycladida. In the first group (figs. 5, 6) the two longitudinal cords and their branches are the most feebly developed, and there is but rarely (Mesostoina, Monotus) a transverse commissure uniting the longitudinal cords. These cords are very large in the Tridadida, where the brain is to be regarded as a simple thickening of them ; in this group there are numerous transverse com missures between the longitudinal nerve cords (fig. 7), and the nerves arising from them and passing to the periphery form a subcutaneous nerve plexus within the muscular coat. Lang has observed a similar nerve plexus in the PolycladicUi, the central nervous system of which differs from that of the Tridadida in that a number of stout nerve cords radiate outwards from the brain as well as the two longitudinal cords ; they are all united together by Fig. 4. Fig. 5. FIG. 4. Plan of an Acoelous Turbellarian. e, eye; m, mouth; ot, otolith; or, ovary ; p, digesting parenchyma ; t, testicular follicles ; vs, vesicula seminalis $ , mule organ of copulation ; $ 9 , common sexual aperture. Pio. 5. Plan of a Khabilocoelous Turbellarian. be, bursa copulatrix ; en, brain; e, eye; </, germarium ; i, intestine; In, longitudinal nerve trunk; r/i, mouth; ph, pharynx ; rs, receptaculum seminis ; ., salivary gland ; t, testis ; v, uterus (containing an egg); v, yelk gland; vs, vesicula seminalis; <t, chitinous copulatory organ; $ 9 , common sexual aperture; be, bursa copulatrix. numerous commissures, and a network is thus formed which extends throughout the body. Fig. 6. Fig. 7. FIG. 0. I an of an Alloiococlous Turbellarian. Lettering as in fig. 5. l- i... 7. Plan of a Triclmlid. ,, anterior, and in, i 3 , paired posterior branches of intestine ; od, oviduct; te, tentacle ; v<l, vas dcferens ; $ . male, and 9 , female copulatory organ. Other letters as in fig. .">.
Oryans. These are represented by tactile organs,