326 



BIOLOGY: GENERAL AND MEDICAL 



habit to its parent. Instead of growing into a long 

 segmented* worm, the egg develops into a short embryo 

 which undergoes a peculiar cystic expansion of the 

 first segment into which the head and neck are withdrawn, 

 forming a distinct parasitic cyst or scolex. Such 

 embryos do not inhabit the intestine, but in some way 

 leave that viscus to encyst themselves in the muscles of 

 the animal, take up a kind of inactive existence, and 

 await the chance of being eaten by man with the flesh 

 of the ox. Should flesh containing such an embryo or 

 scolex be eaten raw the embryo is, of course, killed by 

 cooking it emerges, as it experiences the stimulating 



FIG. 120. Taenia saginata. (Eichhorst.') 



action of the digestive juices, thrusts out its head, attaches 

 itself to the intestinal wall by its cephalic suckers, and 

 develops into an adult worm or strobila. We thus find 

 that for parasites of this class there are two separate 

 cycles of life, lived in two different hosts in alternation. 



Of the human parasites the intermediate hosts are 

 numerous and varied; thus for Tsenia saginata it is the 

 bovine species; of Tsenia solium, the hog; of Dibothrio- 

 cephalus latus, the pike; of Paragonimus westermanii, 

 a mollusk. 



Man is himself the intermediate host of Tsenia echino- 

 coccus, the adult of which infests the dog. 



Blood parasites, shut in the circulatory apparatus, 



