PLATYHELMINTHES 187 



At copulation the penis of one animal is squeezed past the penis of 

 the other into the genital atrium. It cannot enter the oviduct, since 

 this is blocked and so it is received into the stalked gland organ, where 

 the sperm is temporarily deposited. After copulation is finished, the 

 penes are withdrawn and the sperm is transferred from the stalked 

 gland organ to the oviduct. The arrangement of the organs round 

 the genital atrium in the Tricladida varies considerably. In Bdello- 

 cephala, for example, the penis is reduced and, when extruded, does 

 not fill the genital atrium sufficiently to block the opening of the 

 oviduct. In this case a flap of skin has developed which is drawn over 

 the opening of the oviduct, when the penis is extruded. 



After the sperm is transferred to the oviduct, it moves up to the 



*/>• • p. fl.p. geiiat. oil. 



Fig. 149. Longitudinal vertical section through region of the genital atrium 

 in Dendrocoelum lacteum. After Ullyott and Beauchamp. c.o. common 

 opening to exterior; g.o. opening of genital atrium (gen.at.); fl.p. flagellum 

 or penis; nius.or. muscular organ; od. oviduct; p. penis; sp. stalked gland 

 organ (bursa copulatrix) ; ves.sem. expanded portion of vas deferens forming 

 a vesicula seminalis. 



receptaculum seminis at the top, near to the point of discharge of 

 the ova. The ova are fertilized in the oviduct and then move down 

 towards the genital atrium, receiving on the way the products of the 

 vitellaria. On arrival in the genital atrium a cocoon is shaped and made 

 ready to be deposited. When laid it is usually attached to weeds, 

 sometimes by a stalk. 



The parasitic Trematoda and Cestoda are unaffected by the 

 seasons and are perpetually producing eggs. But in the Turbellaria the 

 season of egg laying varies. In some, for example Dendrocoelum lac- 

 teum, the generative system is in full working order all the year round, 

 in others, for example Planaria alpina, the eggs are only produced 

 during the winter months. Mesostomum produces two kinds of eggs 



