PLANARIANS 



81 



simple widening of the oviduct (fig. 8) ; in those forms 

 which possess additional yelk glands the uterus is a simple 

 or paired diverticulum of the atrium genitale (figs. 5, 7). 

 The ova are either surrounded by a more or less hard 

 chitinous shell, or one shell contains a number of ova 

 ("cocoon" of Tricladida and many Polydadida). The 

 Polydadida deposit an egg-string which like that of the 

 Gastropoda consists of a number of eggs bound together 

 by a transparent albumen-like mass. Many Rhabdocoel 

 Turbellarians (e.g., Mesostoma ehrenhergii) produce two 

 sorts of ova, thin-shelled summer ova and thick-shelled 

 winter ova ; the latter are capable of withstanding a 

 considerable amount of dessication, and are deposited in 

 the autumn. The accessory female organs of reproduction 

 are represented by bursse seminales, which receive the 

 semen during copulation and retain it until fertilization 

 is accomplished. A further division of labour is brought 

 about by the presence of two diverticula of the atrium 

 genitale, one of which serves as a bursa copulatrix (fig. 5, 

 he) 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 Tricladida and 

 Polydadida the female efferent duct is often differentiated 

 into a muscular vagina which closely resembles the penis 

 (figs. 7, 8, ? ). 



Finally, the female generative apparatus is furnished 

 with a number of glands which have been termed cement 

 glands, albuminiparous glands, and shell glands. 



The male sexual glands (figs. 4-8, t) resemble the ovaries 

 in being either compact tubular (fig. 5) or follicular 

 (figs. 4, 6, 7, 8) structures. The vasa deferentia (vd) are 

 often widened out into vesiculse seminales (figs. 4, 6, vs) ; 

 or there are special vesiculse 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 Bhahdocoela, 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 Macrorhynchus h.d,golandicus, 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 (Polydadida) and Prorhynckus (Rhdbdocoela), 

 and also the existence of several (2-15) pairs of male 

 copulatory organs and genital apertures in certain Poly- 

 dadida. 



The spermatozoa vary much in form, especially ia the 

 Bhahdocoelida, 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 

 " successive hermaphroditism," 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 is met with only in the Microsto- 

 mida ; 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 pro- 

 cedure 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', m", m'") 

 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 



- Fig. 10. 



Fig. 9. 



Fig. 9. — Microstoma Uneare, Oe., undergoing division. Tliere are 16 individuals, 

 8 witli moTitli apei-tuves, showing tlie buds of the first (m), second (m'), third 

 (m") , and fourth (m"') generation. The fifth generation has not yet acquired 

 a mouth aperture, c, ciliated grooves ; e, eye spots ; z", intestine. 



Fig. 10. — Larva of Yungia aurantica^ L. {Polydadida), with provisional ciliated 

 processes (after A. Lang). 



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 Bhahdocoelida, which are 

 extremely difficult to investigate, and about which hardly 

 any developmental facts are known. The larger fresh- 

 water Tridadida and the Polydadida on the contrary have 

 been recently very fully investigated. The Rhahdocoela and 

 the Tridadida appear to develop directly without any meta- 

 morphosis, while a great part of the Polydadida undergo 

 a metamorphosis 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 larvae with a coat of long cilia apparently belonging 

 to this group have been observed by Ulianin. The seg- 

 mentation of the ovum is total, but unequal ; an epibolie 

 gastrula is formed and the aperture of invagination 

 becomes the permanent mouth of the adult. 



Systematic Arrangement and Mode of Life. — Order 

 Turidlaria. — Platyhelminths with a ciliated integument, 

 a mouth and pharynx, but no anus ; with paired cerebral 

 ganglia and two lateral nerve cords ; sexual organs her- 

 maphrodite ; chiefly free-swimming. 



