INTRODUCTION 



The object of the following exercises is to acquaint the stu- 

 dent with the commoner animal parasites of man and of the 

 domesticated animals, including insect carriers of disease. In 

 studying the parasites here considered, the student should 

 bear in mind the following statements and make observations 

 accordingly. 



A parasite can only be a parasite as it lives directly at the 

 expense of another organism, whether plant or animal. This 

 definition leaves few if any animals out of this category, inas- 

 much as the dependence of animals directly upon other animals 

 or upon plants for food is obvious. But if we restrict this mean- 

 ing to position, living in or upon another animal or plant for 

 purposes of feeding, we come nearer to the thought. But even 

 here there are many organisms which live in or upon living 

 animals or plants, merely sharing their food with them without 

 injuring the host. This we call commensalism. Furthermore, 

 organisms feeding in or upon dead bodies would not be termed 

 parasites, except as they also attack or feed upon living tissue, 

 as occurs in certain flesh flies, e. g. the Texas Screwworm fly 

 (Chrysomia macellaria) , which as a larva may feed upon the 

 flesh of dead or living animals. Thus our definition revolves 

 about the idea of feeding upon living organisms, to which 

 should be added that the host must not be killed before at 

 least the developmental period or larval period of the parasite 

 is completed, otherwise the result would be the destruction 

 of the parasite as well as the host. Parasitism then involves 

 slow death to the host, if fatal consequences are at all in- 

 volved. The definition given by Braun is, viz., "By the term 

 Parasites is understood living organisms which for the purpose 



ri 



