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Rh the uterus—which may either be a dilatation of the common oviduct (vagina) or of the atrium, and may open to the exterior independently (single in Uteriporus, paired in Syncoelidium). In Bdelloura the uterus is said to act as a spermotheca. In addition to these structures, accessory muscular organs are found in Dendrocoelum and developed to a high degree in land Planarians, where they form the so-called adenocheiri and adenodactyli (see von Graff, 1899).

Lastly, the Polyclads offer certain distinctive sexual characters. The ovaries are follicular, very numerous, and the ova elaborate their own yolk (fig. 15). The oviducts open into a chamber which, after receiving a voluminous shell-gland, opens by a muscular bursa to the exterior. No special uterus is developed, but from the point of union of the two egg-chambers a vesicle is given off which may open separately to the exterior (Trigonoporus). The testes are equally diffused and the seminal vesicles usually form a median muscular reversible sac which opens in front of the female genital pore. In Stylostomum, however, this penial organ opens through the mouth, as in certain Rhabdocoelida. Moreover, it may be paired (Thysanozoon) or multiple. Thus in Anonymus twelve or more pairs occur. In Cryptocelides two, four or six may be present, but in this genus they all lie in a common sac. In Polypostia twenty pores occur ranged about the female pore, but the most posteriorly placed of these structures are devoid of a seminal duct. This condition supports the view that in Polyclads the penis was at first a glandular organ probably used for attacking prey and that it has become secondarily connected with reproduction. In confirmation of this conclusion we have the observations of Lang (5) that Yungia stabs the body of other Polyclads with its penis when brought into contact with them. (See Whitman [9].) The genus Laidlavia differs from all other Polyclads in possessing a dorsal genital opening.

Development.—The development of the Planarians is fairly well known. Except for one or two species of Polyclads, development is direct and without metamorphosis; but in Thysanozoon and Yungia the embryo develops eight strongly ciliated lobes which form a circumoral band of larval processes. These have been compared with the girdle of Trochosphere larvae and also with the eight rows of swimming plates in Ctenophores. From the name of their discoverer these girdled larvae are called Müller's larvae (fig. 16).

In the Rhabdocoelida the eggs are usually laid in a shell which has characteristic shapes. Each capsule contains a single ovum and several yolk-cells. Segmentation results in the formation of dislocated megacytes and microcytes. The latter give rise to the epidermis which is laid down in bilateral sheets, the former to the various internal organs. There is no distinction of germ-layers, and the gut is graduallly organized from the mesenchyme, the rest of which gives rise to the parenchyma. The pharynx and the rudiment of the gonads are the first organs to appear (Breslau [13], 1905). The development of the Acoela differs in certain particulars from that of other Rhabdocoelida. The ova contain yolk-granules, and yolk-cells are absent. Groups of such eggs, each with its own shell, are laid in a gelatinous envelope. Each ovum segments into a two-layered embryo composed of a ciliated outer layer and a central syncytium. No trace of a distinct enteron or gut is visible, but as the embryo grows the syncytium becomes differentiated into a more fluid central portion and a firmer peripteral zone. The former, together with the wandering phagocytes, corresponds functionally to the separate gut of other Rhabdocoelida. Pelagic larvae with a coat of long cilia have been identified by Uljanin as belonging to the Acoela.

The development of the Tricladida offers other peculiarities. From four to twenty or more ova are surrounded by several hundred amoeboid yolk-cells in each cocoon. Each egg-cell divides; but, as happens in the capsular ova of certain Mollusca and Oligochaeta, they do not all survive, some being used up as food by the remainder. The segmented ovum becomes dislocated as in some Rhabdocoels, the blastomeres moving apart from one another. The details of organ-formation are still imperfectly understood.

The eggs of the Polyclads are laid somewhat like those of the Acoela in a gelatinous enveloule, each ovum being provided with yolk and an egg-shell which may be operculate. The majority of species go through a direct development. The segmentation of the egg in Discocelis and Leptoplana has been worked out by Lang and his results re-interpreted by Wilson and others (Hubrecht

[12]). In Polyclads a distinction of germ-layers similar to that occurring in the development of Mollusca, Chaetopod-Annelids and certain other Invertebrates, is early apparent. The ovum by unequal segmentation gives rise to megameres and micromeres, and between the two, intermediate cells form one origin for the mesenchyma. The micromeres surround the intermediate and centrally placed macromeres. The latter undergo division into hypoblast cells and yolk-masses. The similarity of cell-lineage in Polyclads and Coelomate Invertebrates, together with the trochosphere-like Polyclad larval form (Müller's larva), have been the two chief arguments in support of the view that this group is a link between the Planarian and Coelomata. It is at present, however, doubtful whether such highly organized animals as Polyclads can be regarded as in any sense ancestral forms. Their relations to other Turbellaria are quite uncertain, and on present evidence it seems legitimate to hold that they are the most highly differentiated division both in embryonic and adult structure.

Order Turbellaria.—Free-living Platyelmia with a ciliated epidermis. A well-developed nervous system and sense-organs concentrated at the anterior end of the body, diffused elsewhere.

Sub-order A. Rhabdocoelida.—Gut syncytial or tubular. Female gonads always compact.

Tribe I. Acoela (fig. 11).—Mesenchyma not differentiated into separate gut and parenchyma. No excretory organs of protonephridial type. A simple pharynx. A median otocyst (statocyst) over the brain. Small, often flattened forms. All marine and many infected by brown or green algal cells. One species parasitic in Echinoderms.

Tribe II. Rhabdocoela (fig. 12).—Gut and parenchyma separate, the former a simple straight sac. Vitellarla usually present. Testes compact. Penis and pharynx often complex, occasionally protruded through a common opening. Marine and fresh-water. Many fresh-water forms infected by algal cells. Typhloplana, Graffilla, Anoplodium, are respectively parasitic in Nephthys, in Gastropods and Holothurians.

Tribe III. Alloeocoela.—Gut and parenchyma distinct. Intestine straight or lobate. Testes follicular. Penis and pharynx simple. One family with otolith. All marine except Plagiostoma lemans (deep-water, Geneva) and the Bothrioplanidae.

Sub-order B. Dendrocoelida.—Large forms with flattened body, branched intestine, follicular testes and follicular ovaries or compact ovaries and yolk-glands.

Tribe I. Tricladida.—Intestine with three main branches. A pair of compact ovaria and numerous yolk-glands connected by a common duct. A single genital aperture. Fresh-water forms: Planaria, Dendrocoelum, Polycelis, common. Peculiar forms in Lake Baikal. Marine forms: Gunda segmentata, Bdelloura (external parasite of Limulus). Terrestrial forms: Rhynchodemus, Geoplana, Bipalium.

Tribe II. Polycladida.—Body leaf-like. Intestine composed of a median stomach with many branched or reticulated coeca; testes and ovaries follicular; genital openings usually separate, the male gonopore preceding the female one. Multiple male gonopores in some forms. All marine and widely distributed; some genera cosmopolitan.

.—(1) L. von Graff (Rhabdocoela, Acoela, Tricladida), Monographie d. Turbellarien (1882), vol. i., (1899) vol. ii.; Die Acoela (1891); (2) ''Arbeiten aus der zool. Institut zu Graz'' (1904, 1905, 1906); (3) “Turbellaria,” in Bronn's Klassen v. Ordnungen d. Thierreichs, vol. ii.; (4) Turbellaria als Parasiten v. Wirthe (Graz, 1904); (5) A. Lang, “Die Polycladen,” Fauna and Flora of the Gulf of Naples, vol. ii. (1884); (6) F. F. Laidlaw (Polyclads) in Zoological Results of Expeditions conducted by Dr Willey, Stanley Gardiner and C. Crossland, Cambridge Univ. Press, and ''Proc. Zool. Soc.'' (1902-1906); (7) Gamble and Keeble (Green cells of Convoluta), Quart. ''Jour. Micro. Sci.'' (1903, 1907); (8) E. R. Pearl (Bionomics of Planarians), ibid. (1903); (9) Whitman (Hypodermic Impregnation), ''Jour. Morphology'' (1890), iv. 361; (10) Hesse (Eyes of Planarians), ''Zeitschr. f. wiss. Zool.'', vol. lxi. (1897); (11) Carrière (ditto), “Die Sehorgane der Thiere” (1885); (12) A. A. W. Hubrecht (affinities), ''Zeitschr. f. Naturwiss.'' (Jena, 1905); (13) Breslau (Development of Rhabdocoels), ''Zeitschr. f. wiss. Zool.'' (1905). Besides these special works, useful general accounts of the Turbellaria will be found in Cambridge Natural History, ii. 1-50; A Treatise on Zoology (Black), iv. 1-42, and the references given by these works.

Class Temnocephaloidea.—This small class of Platyelmia possesses a special interest. It connects the Turbellaria (and in particular the Vorticid rhabdocoela) with the Trematoda. At the same time the Temnocephaloidea present certain peculiar structural features which entitle the class to an independent position.

The name of the class is derived from the digitate tentacles which occur on the anterior or lateral margins of the body. The body measures about 5 mm. in length, and the flattened ventral surface is armed with a sucker. It presents in most genera the appearance of a minute cephalopod, but in Craspedella the posterior