1^6 Appendix. 



labors of the helminthologists finally determined the mode of 

 their origin, and completed the record of their natural history, 

 by showing that, for the full round of their development two 

 anifnals are necessary. The second of these usually stands to 

 the first in the relation of prey or food. The mature parasite 

 lays eggs in the alimentary canal of the first animal. These 

 ova are swept out with the alvine discharges, and through the 

 medium of surface water or herbage, some of them find their 

 way into the alimentary canal of the second animal. Here the 

 ova find conditions favorable to the first stage of their develop- 

 ment, and they are now provided with a boring apparatus by 

 which they make their way through the walls of the canal, and 

 travel long distances, finally to ensconce themselves in the solid 

 tissues where they become encysted. The second animal being 

 killed, its flesh is eaten by the first. The cyst wall is digested, 

 and the cysticercus, thus freed from its environment, now finds 

 the appropriate nidus for the final stage in its development. 

 Thus each taenia has its own cysticercus whose distinctive 

 characteristics can be recognized under the microscope, and 

 furthermore the taenia, peculiar to one species of animals, is 

 never found infesting any other species.* 



One can never sufficiently admire the splendid patience of 

 such men as Diezing, Kuchenmeister, Haubner, Von Siebold, 

 Leuckart, Van Beneden and others, who almost literally devoted 

 their lives to these studies. The details of their experiments, 

 both on man and on the lower animals, and their cautious, long- 

 continued, and at times unpromising researches, form one of the 

 most entertaining as well as instructive chapters in the whole re- 

 cord of natural history study. f These labors, it is true, were 



* Van Beneden's little book, Animal Parasites and 3fessmates, puhMshed 

 since tliis lecture was given, fui-nishes for the English reader an excellent ac- 

 count of the development and migrations of these entozoa. New York : D. 

 Appleton «fe Co., 1876. 



t An amusing illustration of the precision of the results obtained by these 

 investigators is found in the well-known narrative of Van Beneden* in his 

 monograph upon Intestinal Worms. For the purpose of illustrating the 



* Van Bexeden : Mhnoire sur les Vers Intestinaux, Paris, 1858, p. loo, and 

 Animal Parasites and Messmates, pp. 71 and 222. 



