Phyla Platyhehninthes and Nemertina 417 



deferens. The vas deferens is usually much coiled, and often has an 

 enlargement for sperm storage, the seminal vesicle. The vas deferens 

 ends in an intermittent organ, the cirrus which is retractile into the 

 cirrus sac. The cirrus opens into a cup-shaped structure, the genital 

 atrium, which has a pore on the surface. 



ONGITUOINAL 

 \ NERVE 



PBOGLOTTIO. 



Fig. 136. — The pork tapeworm, Taenia solium. A, Scolex; B. mature proglottid; 

 C, gravid proglottid; D, egg with oncosphere (six-hooked embryo) inside; E, cysti- 

 cercus (bladder worm) that encysts in muscle; F, cysticercus with scolex everted. 



The female system has a single or multilobed ovary. As in the 

 fluke, the ovary is connected by a short oviduct with the ootype. Also 

 entering the ootype is the duct of the yolk gland and the vagina. The 

 vagina is connected to the same genital pore used by the male system, 

 and sperm probably pass down the vagina to the ootype. In the ootype 

 the egg is fertilized, the yolk is added from the yolk gland, and the 

 shell from the shell gland that is also associated with the ootype. From 

 the ootype, the fertilized egg passes into the large uterus. Self-fertili- 

 zation or fertilization between adjacent proglottids may occur. As 

 the eggs are produced and passed into the uterus, the uterus enlarges, 

 gradually displacing the other structures which soon degenerate. As 

 a result, the mature gravid proglottid is nothing but a sac of eggs. 



