VI AUTHORS PEEFACE. 



to neglect matters which at first sight appear further removed from 

 his department than from that of the Zoologist. 



But just as little is it permissible for the latter to forget that the 

 knowledge of the life-history of animals, after which he strives, is to 

 be obtained by the investigation not only of their organization and 

 development, but also of the position which each species occupies in 

 the economy of Nature, — in the present instance of the attitude which 

 the Parasite assumes towards its host. 



But few decades have passed since the full extent and the 

 significance of these relationships have been made clear to us. It was 

 only with the introduction of Helminthological experiment that a 

 new path was opened to the field of knowledge, and we Zoologists 

 gratefully recognise that the first impetus to the brilliant discoveries 

 which our science has to show was the work of a Physician, and we 

 rejoice that at the present day Medicine takes an active part in the 

 prosecution of these studies. This partnership in the work ensures 

 further progress in the future, which is the more important, since 

 our knowledge of the Parasites of Man in particular has in no respect 

 reached a satisfactory condition. Numerous weighty questions still 

 await their final solution. 



As to the part which I have personally taken in the cultivation 

 of the science, it may be passed over with the remark that I have 

 devoted my labours to it for a period of more than thirty years. If 

 my efforts have in many respects been crowned with success, I owe it 

 mainly to the long period during which I have followed up the 

 solution of the problems in hand. The number of animals used for 

 Helminthological experiments amounts to many hundreds, and much 

 larger is the sum of the Parasites investigated. 



What I offer to my readers, then, is the result of a prolonged and 

 minute investigation, and my work contains little which does not rest 

 upon the basis of personal observation. 



Although my book is devoted mainly to the Entozoa infesting 

 Man, it offers an almost complete survey over the present state of 

 that part of Zoology which treats of Parasites. The first section 

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