FLATWORMS 



worms; the copulatory organ may be inserted into the vagina of the same 

 proglottid, or into the vagina of a different proglottid in the same strobila if 

 the worm is folded upon itself. Cross-fertilization may occur during similar 

 contact if two or more worms are living in the same host. As the cells com- 

 posing the ovaries, testes, and yolk glands in each proglottid are utilized, 

 the eggs thus formed accumulate in the uterus, which grows as the other 

 parts of the system dwindle. Eventually the greater part of the proglottid is 

 occupied by the branching uterus containing tens of thousands of eggs enclos- 

 ing six-hooked embryos, which have developed from the zygotes (Fig. 11.19). 

 The ripe proglottids become detached, pass out of the host with the feces, and 

 disintegrate. The embryos may thus be liberated on the surface of the 

 ground, where they can survive protected by the shell for months and may be 

 distributed widely like encysted protozoans. 



Within the eggshell, development proceeds no further than the six-hooked 

 embryo (hexacanth) stage unless the egg is ingested by another host; in the 



Fig. 11.19. Stasjes in the life cycle of Taenia solium. .4, egg containing hexacanth embryo. 

 B, hexacanth after emergence in gut of intermediate host. C, four cysticerci in pork, pro- 

 gressively dissected to show cyst wall, inner envelope, and invaginated scolex. /), cvsticercus 

 evaginating in gut of final host. (Redrawn from W. Stempell, 1926, ^oologie im Grundnss.) 



337 



