418 The Animal Kingdom 



The Life Cycle of Taenia Solium. — One common tapeworm that 

 affects human beings, in all parts of the world is Taenia solium, the pork 

 tapeworm. The eggs develop in the mature proglottid which breaks 

 off and passes out with the feces of the host. Under favorable condi- 

 tions, the proglottid ruptures, the eggs escape, and are dispersed on 

 the ground. Here the larvae within the ^gg continue their develop- 

 ment until a six-hooked embryo or oncosphere (Fig. 136,Z)), is pro- 

 duced. There is no further development unless the ^gg is picked up by 

 the proper intermediate host, usually a pig. When eaten by a pig, 

 the digestive juices dissolve the shell, and the six-hooked embryo bur- 

 rows through the wall of the digestive tract into the circulatory sys- 

 tem. Here it is carried to the voluntary muscles where it forms a 

 cyst. This cyst enlarges, filling with fluid, and is primarily a hollow 

 sac or bladder known as a bladder ivorm or cystic ercus (Fig. \Z6,E 

 and F). Projecting into the hollow area is a papilla in which a 

 scolex with hooks and suckers develops. Further development stops 

 unless infected uncooked pork is eaten by man, in which case, the 

 scolex everts, the bladder disappears, and a new worm develops. 

 This worm soon starts budding off proglottids, and may attain as much 

 as 10 feet in length. 



One of the interesting aspects of this tapeworm is the fact that 

 man is the only known definitive host for the adult ; however, the 

 cysticercus may occur in a great number of animals, including man, 

 monkeys, camels, and dogs. The presence of the cysticercus in volun- 

 tary muscle or in delicate tissues such as the eye and brain may cause 

 very serious conditions. Removal is extremely difficult. Self-infection 

 may occur by uncleanly habits or by reverse peristalsis carrying the 

 proglottids into the stomach. Within the stomach, the eggshell could 

 be digested off. 



Other Important Tapeworms. — The beef tapeworm of man. 

 Taenia saginata, has a life history practically identical with that of the 

 pork tapeworm except that the secondary hosts are cattle. These tape- 

 worms may attain lengths as great as 50 feet with 20 to 30 feet being 

 more common. Ordinarily man does not serve as an accidental sec- 

 ondary host. 



Dibothriocephalus latiis (formerly Diphyllobothrium latum) (Fig. 

 \37,A-E) is the largest known tapeworm to infect man. Some of these 

 attain lengths up to 60 feet and may have 4,000 proglottids ! The infec- 

 tion occurs from eating uncooked fish that carry the parasites. As is 



