AUTHOE S PREFACE. Vll 



contains a general natural history of these remarkable animals, 

 intended to give a clear exposition of the phenomena of Parasitic 

 life in its various forms, as well as to narrate the history of our 

 knowledge of them. And similarly there is prefixed to the special 

 account of the various species a general sketch of the structure and 

 life-history of the groups to which they belong. This course was 

 adopted not only for purely scientific reasons, not only in order that 

 the individual facts might be fully treated in connection with related 

 phenomena, but also because by this means alone was it possible to 

 supply, by well-grounded hypothesis and inductive reasoning, the 

 gaps in our experience. The basis of our knowledge must be as 

 extensive and as profound as possible, in order that the origin and 

 nature of Parasites may be treated clearly and satisfactorily. 



By this mode of dealing with the subject I hope to have met the 

 wants of those who are actuated by no interest in the Parasites of 

 Man in particular. Here I refer chiefly to the Veterinary Surgeon 

 and Cattle Breeder, who, in a summary of all that is known regarding 

 the life-history of Parasites, will find the means of becoming more 

 closely acquainted with those specially important Entozoa of our 

 Domestic Animals which also infest Man. 



In leaving out of consideration the Therapeutic treatment of 

 Parasitic Diseases, I have followed the advice of one of our greatest 

 medical authorities, and I did so the more readily, since, owing to the 

 lack of personal experience in this matter, I could only have re- 

 capitulated the works of others. Correspondingly greater prominence 

 has, however, been given to those Hygienic principles which the study 

 of Parasites gives us for the protection of society and its material 

 interests, and which demand the more attention since they have 

 hitherto been insufficiently practised. It is in this connection that 

 the importance of modern Helminthology is most conspicuous ; for 

 nowhere is it more true that " prevention is better than cure," than in 

 the case of Parasitic Diseases. It is sufficient to point, by way of 

 illustration, to the Hydatid Tumours, Liver-Rot, and Trichinosis. 



In spite of the importance attributed to the medicinal aspects of 



