176 



METAZOAN PHYLA 



203. Life History of a Tapeworm. — The life history of a cestode 

 (Fig. 87) also involves two hosts, the final host becoming infected with 

 the parasite through eating the intermediate host. A typical life history 

 is that of the beef tapeworm, Taenia saginata (Goeze), the adult of which 

 is found in the human intestine, and which passes through the following 

 steps in its life history: 



Fig. 87.— Diagram showing the life history of Taenia saginata. A, the adult tapeworm 

 (reduced), from intestine of man. B, the egg (much enlarged), containing six-hooked 

 embryo. Such eggs pass out of the host in the feces and do not hatch until eaten by 

 cattle. C, young larva (cysticercus) from muscles of infected cow. D, later stage in the 

 development of the cysticercus. E, cysticerci (about natural si2e) embedded in beef. 

 F, diagram of cysticercus (enlarged) showing invaginated scolex. G, cysticercus showing 

 evaginated scolex and early proglottids. {B, C, D, G from Leuckart. " Parasiten des Men- 

 schen"; F, from Faust, "Human Helminthology," figures arranged by H. W. Manter.) 



1. The egg cell is produced in the ovary, passed into the oviduct, 

 fertilized, supplied with yolk, inclosed in a shell, and carried to the uterus. 

 In the uterus there develops in this egg a six-hooked embryo. As the 

 proglottid becomes ripe and passes out of the body of the host with the 

 feces, it carries with it thousands of embryos, still inclosed in the egg 

 shells. 



