DEVELOPMENT OF PARASITES \J 



may destroy an entire generation of an insectivorous species of bird 

 within a small district. 



Naturally it does not follow that direct development in the 

 cestodes is altogether lacking. The researches of Grassi (1889) have 

 furnished an example in Hyjuenolepis (T&nia] mnrina, which shows 

 that development may sometimes take place without an intermediate 

 host, notwithstanding the retention of the cystic stage. It was found 

 that the oncospheres of this species, introduced into rats of a certain 

 age, after a time grow into tapeworms without leaving the intestine, 

 but not directly, for they bore into the intestinal wall, where they 

 pass the cystic stage, the cysts afterwards falling into the intestinal 

 lumen, where they develop into tapeworms. The recent experiments 

 of Nicoll (1911) show that the larval stages of Hymenolepis murina 

 also occur in the rat-flea, Ceratophylhis fasciatus. 



Important observations were soon made on the remaining 

 groups of helminthes. The discussion on the origin of parasites 

 soon became confined to the helminthes. Amongst the Nematoda, 

 it had long been known that encapsuled 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 demon- 

 strated 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 

 encapsulating 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 encapsuled nematodes could no longer be regarded as the 

 result of straying. Subsequently, R. Leuckart worked out, more 

 or less completely, the history of the development of numerous 

 nematodes, or pointed out the way in which further investigations 

 should be made. It has been found that in nematodes far more 

 frequently than in other helminthes, the typical course of develop- 

 ment is subject partly to curtailment and partly to complications, 

 which sometimes considerably increase the difficulties of investigation 

 and have hitherto prevented the attainment of 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 Acauthocepliala and Linguatulida. Of 



