CESTODA 



201 



the Turbellaria greatly elongated and used for egg storage, while the 

 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 



In.eivc.ca. tr.exc.ca 



vas de. 



y.'gl. sh.gl 



ov. 



Fig. 158. Diagram of a ripe proglottis of Taenia solium, x about 10. From 

 Cholodkowsky. In.exc.ca. longitudinal excretory canal; tr.exc.ca. transverse 

 excretory canal; vas de. vas deferens; vag. vagina; ov. ovary ;y.gl. yolk gland; 

 sh.gl. shell gland; ut. uterus; t. testes; In.n. longitudinal nerve. 



the exterior in the Malacocotylea but into the gut in the Hetero- 

 cotylea. 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. 159 and 160.) 



The life history of a cestode is a complicated combination of sexual 

 and asexual reproduction. One, two or three hosts may be necessary. 

 The egg passes to the exterior with the faeces. It contains inside it 

 an embryo armed with six hooks called an "onchosphere". The egg 

 case takes different shapes; in Bothriocephalus latus the covering of 

 the embryo is ciliated, in Dipylidiiim caninum the ciliary covering is 

 replaced by>an albuminous coat with a chitinous lining inside, in most 



