232 THE INVERTEBRATA 



vagina of the Cestoda is the same, but the relation of the *' vagina" of 

 the Heterocotylea or the ** uterus" of the Cestoda remains at present 

 obscure. 



If the vagina of the Cestoda is homologous with the uterus of 

 the Trematoda, the uterus of the Cestoda, which is a single duct, 

 may correspond with the vagina of the Trematoda, which is however 

 a paired structure. The homologies of the ducts in the Trematoda are 

 further complicated by the presence of Laurer's canal, a duct leading 

 out of the ductus communis and opening to the exterior in the 

 Malacocotylea but into the gut in the Heterocotylea. The bursa 

 copulatrix and the muscular pear-shaped organ, which open into the 

 genital atrium in the Turbellaria, are accessory reproductive organs 

 which are probably not represented in the parasitic forms. (See 

 Figs. 165 and 166.) 



