212 



THE INVERTEBRATA 



genital atrium and opening, so that the opening of the oviduct into 

 the genital atrium is blocked and no sperm can enter or ova escape. 

 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 are withdrawn and the sperm is transferred from the stalked gland 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 Bdellocephala, for 

 example, the penis is reduced and, when extruded, does not fill the 

 genital atrium sufficiently to block the opening of the oviduct. In 



vc's^em. 



mini.0K 



g.o. 



Fig- 154- 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 

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

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

 a vesicula seminalis. 



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 

 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 sea- 

 sons 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, 



