764 TEXTBOOK OF ZOOLOGY 



the one at the end of the strobilus farthest from the scolex. The 

 youngest proglottids contain no recognizable structures, except the 

 paired longitudinal nerve cords and longitudinal excretory vessels 

 which run the full length of the strobilus, and a transverse excre- 

 tory vessel in each proglottid. As the proglottids become older and 

 are pushed farther away from the scolex, the reproductive organs 

 begin to develop; each proglottid develops a complete set of both 

 male and female reproductive organs; when these become mature 

 and ready to function, the proglottid is a mature proglottid. Each 

 mature proglottid is capable of copulating with itself, by bending 

 the cirrus down into its own vagina, or it may copulate with another 

 mature proglottid of another tapeworm if one is present close by. 

 After copulation the male organs begin to degenerate, the uterus 

 becomes filled with fertilized eggs and takes up more and more 

 space, then the ovaries, vitellaria, and other female organs degen- 

 erate and leave the uterus to occupy nearly the whole proglottid; 

 in this condition the proglottid, now hardly more than a sack of 

 eggs, is called a gravid proglottid. The gravid proglottid at the 

 extreme end of the strobilus breaks off and passes out of the host's 

 intestine with the feces. It continues to live and crawl slowly 

 through the feces like an independent animal for a few hours, then 

 usually dies, but the embryos within the proglottid remain alive 

 much longer. If gravid proglottids or separate eggs are eaten by 

 a hog, the six-hooked hexacanth larva hatches in the hog's intestine, 

 bores through the intestinal wall, and migrates to other parts of 

 the body where it changes into a bladder-Avorm or cysticercus, which 

 is a saclike larva with an inverted scolex. The cysticerci remain 

 in the flesh of the hog until the pork is eaten by man ; under the 

 influence of human digestive juices the cysticerci become everted 

 so that the scolex is on the outside of the saclike part, then the 

 scolex attaches itself to the wall of the human intestine, proglottids 

 begin to bud off from the neck, and an adult tapeworm is formed 

 within a few weeks. 



Taenia saginata, the beef tapeworm, has a similar life history, but 

 uses cattle instead of hogs as intermediate hosts. Taenia serrata, 

 a common dog tapeworm often used as a laboratory specimen, is 

 very similar to the two human species in structure and life cycle. 



