40 THEOEY OF THE ORIGIN OF PARASITES. 



of experiment, tliat encysted Distomes grew mature directly after 

 their migration from one host to another, as von Siebold had believed, 

 and that in tliis way a migration to another host, or another organ, 

 was necessary in the Trematodes, before they could attain to sexual 

 maturity. Although hitherto the various stages of development of 

 the same species had not been experimentally proved,^ as in the 

 Cestodes, we must regard our knowledge as having been completed 

 by the experimental verification of this fact, and especially by Zeller's 

 beautiful researches into the life-history of the ectoparasitic forms, 

 especially Polystomum integerrimum of the frog,^ which have com- 

 pleted our knowledge in another direction. 



The parasitic Nematodes resisted investigation for a very long 

 time. In the year 1863, when the first volume of my work on para- 

 sites appeared, I was only able to mention a single Nematode besides 

 the Grordiaceae just referred to (p. 35), whose developmental history 

 was thoroughly known. This was Trichina spiralis, which Virchow's^ 

 and my own* researches showed to be developed in the intestines of 

 rabbits, swine, and other Mammalia into a sexual form, which had 

 previously escaped attention. The young of this worm are produced 

 viviparously, and wander away from the intestine, becoming finally 

 encysted in the well-known way in the muscles. Since this time, 

 however, our knowledge has been considerably advanced by the 

 observations recorded in the second volume of my manual. "We are 

 now acquainted not only with the life-history and migrations of the 

 Acanthocephala, but also of numerous thread-worms belonging to 

 different groups, and can show experimentally that the germs are 

 extruded from the body of the host, and give rise to young, which, 

 at a particular period of their existence, return to the body of their 

 definitive host. Many of my further observations have greatly 

 widened our conceptions of the parasitic mode of life, and have 

 especially placed beyond a doubt the fact that a change of host is by 

 no means a necessity in tlie life-histo^-y of all Entozoa. In many 

 Nematodes, as we shall learn in the next chapter, the young stages 

 are passed in an entirely free existence, and often (especially in 

 certain Strongylidse) under conditions similar to those enjoyed by 

 free-living animals. The life-history of the Pentastomida shows, 

 however, that a migration from one host to another is not confined 



' Even the experiments of Wagner upon Distomum cygnoides leave a considerable gap 

 in respeut of its migration into the body of the frog, " Beitrag zur Entwickelungsgeschiohte 

 der EingeweidewUrmer," p. 29 ct seq. : Haarlem, 1857. 



^ Zeitsckr.f. loiss. Zool., Bd. xxii., p. 1, 1872, and Bd. xxvii., p. 238, 1876. 



' Archivf. pathol. Anat., vol. xviii., p. 330, 1860. 



' Zeitschr. f. rationelle Medicin, Bd. viii., pp. 259 and 335, 1860. 



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