1 8 THE ANIMAL PARASITES OF MAN 



leaving the intestine, but not directly, for they bore into the 

 intestinal wall, where they pass the cystic stage, the cysts after- 

 wards falling into the intestinal lumen, where they develop into 

 tapeworms. 



Important observations were soon made on the remaining 

 groups of helminthes. The discussion on the origin of para- 

 sites soon became confined to the helminthes. Amongst the 

 Nematodes, it had long been known that encapsulated forms 

 existed that had at first been regarded as independent species, 

 but very soon they were pronounced to be immature forms, 

 in consequence of their lack of sexual organs. Though Dujardin 

 and also v. Siebold regarded them as " strayed " animals, 

 v. Stein (1853) very promptly demonstrated that the progeny 

 of the nematodes were destined to travel by discovering a 

 perforating organ in the larval nematodes of the mealworm. 

 This was. first experimentally confirmed (1860) by R. Leuckart, R. 

 Virchow and Zenker, all of whom succeeded not only in bringing to 

 maturity the muscle trichinae (known since 1830) in the intestine 

 of the animals experimented upon, but were likewise able to 

 follow the migrations of the progeny. Of course, the encapsu- 

 lating brood remained in the same organism, and in this respect 

 deviated from the broods of other helminthes which escape into 

 the outer world and find their way into other animals, but the 

 encapsulated nematodes could no longer be regarded as the result 

 of straying. Subsequently, R. Leuckart, more or less completely, 

 worked out the history of the development of numerous nematodes, 

 or pointed out the way in which further investigations should be 

 made. We have learnt that in nematodes, far more frequently 

 than in other helminthes, the typical course of development is 

 subject partly to curtailment and partly to complications, which 

 sometimes considerably increase the difficulties of investigation 

 and have hitherto prevented us from reaching a definite conclusion, 

 though the way to it is now clear. 



In a similar manner the works of R. Leuckart have cleared 

 up the development of the Acanthocephala and Pentastomes. Of 

 course, much still remains to be done ; so far, we do not even 

 know all the helminthes of man and of the domestic animals in 

 all their phases of life, and still less is known of those of other 

 animals. We are indebted to the discoveries of the last fifty 

 years for the knowledge arrived at, though comparatively few 

 names are connected with it ; the gross framework is revealed, 

 but the gaps only have been filled up here and there. However, 



