PHYLUM PLATHELMINTHES 121 



The taeniae which dwell in man have a cysticercus, while one 

 of the commonest tapeworms of dogs (Dipylidium) and of rats 

 (Hymenolepis) have a cysticercoid larva. Some of the worms 

 having a cysticercus undergo asexual reproduction in the larval 

 stage, developing several scolices within a single bladder (genus 

 Coenurus) or at times budding off a series of daughter cysts each 

 with one or more scolices (genus Echinococcus). 



Life History. — As an illustration of cestode development the 

 life cycle of Taenia saginata, the beef tapeworm of man, will be 

 outlined. The mature individual of Taenia saginata occurs in the 

 intestine of man. As the older proglottids become gravid, they 

 become detached from the end of the chain and pass out of the 



Fig. 68. — Schematic drawings of cestode larvae. A, plerocercoid; B, cysticer- 

 coid; C, cysticercus. (Orig.). 



body along with the feces. These isolated proglottids have inde- 

 pendent powers of movement and may crawl away from the feces 

 onto grass or other vegetation where they might be taken into 

 the stomach of a grazing cow, along with the grass. In the 

 stomach of the cow, the walls of the proglottid are digested 

 away and the shells of the embryos open, thus liberating the 

 onchospheres in the digestive cavity of the cow. Each oncho- 

 sphere by the action of its hooks bores into the wall of the diges- 

 tive tract and may enter the blood stream by which it is carried 

 to remote parts of the body. Especially in the muscular and 

 connective tissues, the onchospheres come to rest and undergo a 

 transformation to large saclike structures, the cysticerci, each 

 bearing an inverted scolex. Each cysticercus is surrounded by a 

 cyst wall formed by the action of the surrounding host tissues. 

 Here it lies without going further in its development unless 

 introduced into the body of another animal which is suitable as a 

 host. Cysticerci in thoroughly cooked infected beef are harmless. 

 If beef containing the living cysticerci is eaten by man, the 

 digestive action liberates the cysticercus from its confining cyst, 



