Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/184

Rh 174 until fertilization is accomplished. A further division of labour is brought about by the presence of two divertieula of the atrium genitale, one of which serves as a bursa copulatrix (fig. 5, be) and the other as a receptaculum seminis (rs) in the same sense as the equivalent organs of insects. In the place of a special receptaculum seminis the efferent duct of the ovary is often (Mesostomida) metamorphosed into a chamber to contain the semen. In the Tridndida and Polydadida the female efferent duct is often differentiated into a muscular vagina which closely resembles the penis (figs. 7, 3, 9 ). Finally, the female generative apparatus is furnished with a number of glands which have been termed cement glands, albumini- parous glands, and shell glands. The male sexual glands (tigs. 4-8, t) resemble the ovaries in being either compact tubular (fig. 5) or follicular (figs. 4, 6, 7, 8) struc tures. The vasa deferentia (rd) are often widened out into vesicuhe seminales (tigs. 4, 6, vs) ; or there are special vesicuhe seminales present, formed by a portion of the penis (fig. 5, vs). In the male organ of copulation there is frequently found in addition to the spermatozoa an accessory granulated secretion produced by special glands, but of unknown function. The muscular penis, especially in the Rhabdoccda, has a number of chitinous spines and hooks which serve to assist the animal in maintaining a firm hold during copulation, but also in capturing and retaining its prey. In Macro rhynchus helgolandicus, Gff., there is a peculiar poison dart connected with the male copulatory organ which only serves the latter purpose. Very remarkable is the opening of the penis into the mouth cavity in Stylostomum (Poly clad ida) and Prorhynchus (Rhabdocoela), and also the existence of several (2 -15) pairs of male copulatory organs and genital apertures in certain Polydadida. The spermatozoa vary much in form, especially in the IthaMo- ccelida, where frequently the species of one and the same genus are distinguished by the different form of the spermatozoa. Copulation in the Turbellarians is generally reciprocal; only in those cases where both summer and winter ova (see above) are formed do the former arise from self-fertilization ; the latter are the result of the copulation of two individuals. The fertilization of the ova always takes place in the atrium genitale. Many Turbellarians, especially the Accela, display the phenomenon known as &quot; successive her- maphroditism,&quot; the male organs of an individual attain to maturity first, and the female organs become ripe subsequently. During copulation, therefore, one individual is physiologically a male and the other a female. Asexual generation&quot; is met with only in the Microstomida ; it takes the form of transverse division accompanied by budding. The posterior third of the body becomes separated off by a septum running from the gut to the integument and an external furrow corresponding to this ; this part of the body grows in length until it equals the anterior portion. By further repetition of this double procedure of separation and equalization there, chains of 4, then 8, 16, and 32 buds are formed, which remain attached (fig. 9), and, although fresh mouth apertures (m, in&quot;, m &quot;) have been formed, are still in communication by the intestinal lumen ; this becomes closed before or after the several buds break off from their connexion with each other. Throughout the whole summer chains of zooids are met with ; in autumn this asexual division probably ceases to occur ; the several individuals become sexually mature, separate from each other, and lay eggs which remain quiescent during the winter and in the spring develop into fresh individuals reproducing asexually. Development. The study of the development of the Turbellarians is unfortunately not very far advanced, particularly among the small Rhabdoccelida, which are extremely difficult to investigate, and about which hardly any developmental facts are known. The larger freshwater Tridadida and the Polydadida on the contrary have been recently very fully investigated. The Rhabdocoela and the Tridndida appear to develop directly without any metamor phosis, while a great part of the Polycladula undergo a metamor phosis and pass through a larval condition, during which they are furnished with provisional ciliated processes (fig. 10) ; the Accela have also a free larval form ; pelagic larva; with a coat of long cilia apparently belonging to this group have been observed by Ulianin. The segmentation of the ovum is total, but unequal ; an epibolic gastrula is formed and the aperture of invagination becomes the permanent mouth of the adult. Systematic Arrangement and, Mode of Life. Order Turlcllaria. Platylielminths with a ciliated integument, a mouth and pharynx, but no anus ; with paired cerebral ganglia and two lateral nerve cords ; sexual organs hermaphrodite; chiefly free-swimming. Sub-order A. Rhabdoccelida. Of small size ; body cylindrical or depressed ; without an intestine, or with a simple unbranched int -stine; the female genital glands always compact, not follicular; genital apertures single or distinct. Tribe I. Accela (fig. 1, a). &quot;With a digestive parenchyma not differentiated into intestine and parenchyma proper ; with no nervous system or excretory organs ; sexual organs hermaphrodite, with follicular testes and paired ovaries : generally without a pharynx, but having otoliths ; all the forms marine. Many quite Hat, with the lateral margins bent down towards the ventral surface (Convolnta), frequently with brown or green parasitic algas in the parenchyma. Tribe II. Rhabdocoela (fig. 1, 1). Intestinal tract and paren chyma separate ; nervous system and excretory organs present ; with compact testes and female generative glands (ovaries or separated germarium and yelk glands) ; with a complicated pharynx, but generally without otoliths. Numerous forms, freshwater and marine ; the gnjms Prorhynchus (two species) also in damp earth. The Microstomida (fig. 9) propagate asexually. I-ig. ! . Fig. 10. FIG. Q.Microstoma Jineare,Qe., undergoing division. There are Ifi individuals, 8 with mouth apertures, showing the buds of the first (m), second (m ), third (m&quot;), and fourth (&quot;) generation. The fifth generation has not yet acquired a. mouth aperture, c, ciliated grooves; e, eye spots ; i, intestine. FIG. 10. Larva of Yungia aurantica, L. (1 olycladida), with provisional ciliated processes (after A. Lang). Freshwater forms mostly belong to the families Mesostomida and J orticida, some of which contain green parasitic alga 1. Marino. forms include representatives of these two families and of the Proloscida (with a tactile proboscis). Of the family ]&quot;orticida, the genera Graffi.Ua and Anoplodium are parasitic, the former in Gastropods the latter in Echinoderms (Holothurians). Tribe III. Alloiociela (fig. 1, c). Intestinal tract and parenchyma separate ; nervous system and excretory organs present ; with folli cular testes and compact female glands (as in the Rhabdoazla) ; pharynx similarly developed as a shorter or longer sac. One family (Monotidci), with otoliths. All the species marine, with one excep tion, Plagiostoma lemani, which lives in the deep water of the Alpine lakes. Sub-order B. Deiulroccclida. Large forms, with a flattened body, branched intestine, follicular testes and follicular yelk glands or ovaries ; without otoliths. Tribe I. Tricladida. Bod.y elongate ; intestine with three main branches uniting to open into a cylindrical retractile pharynx ; with follicular testes, two round germariums, and numerous yelk follicles, with a single sexual aperture. Plauaria, Dendroccelum, Puhjcdis (fig. 1, g) are inhabitants of fresh water (with great power of reproduction). Terrestrial forms (fig. 1, e,f) of leech-like shape, especially met with in the tropics (only two European species Rhyn- ckodemus tcrrcstris and Geodcsmus Mlineatus) ; marine forms Gunclti (characterized by a metameric structure), Bdclloura (external para site of Limulun). Tribe II. Polydadida (fig. 1, d). Body leaf-like, thin, and broad, with numerous branched or retiform intestinal cocca which unite to form a central tube (stomach) ; with follicular testes and folliculnr ovaries, with two separated genital apertures, the male in front of the female ; without (Acolylca) or with (Cotylea) a sucker situated behind the female generative opening. All marine. Literature. The most recent works, which also contain a full account of wht lias gone before, are the following :/MaMoefa.L. v. Graff, Monographie der Turbetlarien: 1. Rhabdoccelida, Leipsic, 1SK2, with. plates. Marine ar.d Freshitater Tridadida. A. Lunif, &quot; D&amp;lt; r Bau von Giinda teymentata und die Verwandtschaft der I latyhehninthcn mit Ccelcnteniten und Ilirudineen,&quot; in Mittli. Xool. Stat. ffeapel, vol. iii., 1881; Kl. Metschnikoff, &quot;Die Erobryologle von I lanaria polycln-oa&quot; in ZriUehr.f. tciss. ZooL, vol. xxxviii., 18S3 ; Isao Jijima, &quot; L ntersuchuiigen liber tlcn Bau und die Entwickelungggeschichte der SUss- wassei -Deiidrocoelen,&quot; in Zeitichr f, tciss.Zoo!., vol. xl., 1884. Land I lanarians.