10 



it. In the illustrations of the species certain features which present 

 specific differences have been constantly drawn. Peculiar features of 

 anatomy and development have also been illustrated. The entire de- 

 velopment of any species from the uuimpregnated ovum to the adult 

 form is not illustrated, but one species may show the developing ova, 

 another the embryo, and still another small and adult forms, and thus 

 the entire development of many of the species can be well understood. 

 Especial attention has been devoted to representing certain organs of 

 economic Importance, i. e., those organs which are immediately con- 

 cerned in injuring the tissues of the sheep. The majority of the draw- 

 ings were made from nature by Mr. W. S. D. Haines, and the others by 

 Dr. George Marx, both artists connected with this Department. The 

 excellence of their work shows for itself. Where material for original 

 illustration has been unavailable, figures chosen from the leading 

 text-books on the subject under consideration have been copied, and 

 due recognition of the source acknowledged in the description at- 

 tached. For the accuracy of these drawings the author alone is respon- 

 sible. He believes that all the anatomical details are accurate, but such 

 is the difhculty of seeing the minuter details that some of the latter are 

 omitted. As their presentation belongs properly to a more specific 

 investigation than this their absence will scarcely be noticed. It has 

 been the constant endeavor of both the artists and the author to make 

 technically perfect drawings, and at the same time present the subject 

 so clearly to the eye that not only a novice may, by the aid of a small _ 

 magnifying glass, be able to determine the species, but that the scien-* 

 tist may also use the work profitably in subsequent investigations. 



The text devoted to each species is intended to contain a general 

 description of the parasite, its life history, the way it causes disease, 

 the disease produced and mode of treatment, both preventive and re- 

 medial. Many of the specific descriptions are technical. To the be- 

 ginner, who can identify the species by careful comparison with the 

 figures, these are unnecessary, but as he advances in their study the 

 meaning of the technical descriptions will become more apparent and 

 useful. In a work of this character such technical specific descriptions 

 are unavoidable. To the scientist they are absolutely necessary. 

 Wherever possible the complete life-history of the parasite is described 

 and illustrated ; unfortunately, however, the species whose life-histories 

 are positively known are too few. Although the life-histories of the 

 majority of the worms seem very evident, still the evidence upon which 

 they are based is not deemed entirely conclusive by scientists. So skep- 

 tical are the majority of this guild that rigorous proof alone seems to 

 satisfy them, and this is particularly the case when the views set forth 

 in regard to either of the species are at variance with pre-existing 

 opinions. 



Eigorous demonstration of the various stages in the life-history of a 

 parasite demands that its eggs or embryos shall be fed to an uninfected 



