- INTRODUCTION xiii 



situation will be greatly augmented. Thus the effect of parasi- 

 tism on the host depends both on the number and position of 

 the parasite. 



Effect of parasitism on the parasite. All parasites are 

 more or less specialized in the direction of their habits, e. g. fleas 

 are laterally compressed to effect ease of motion between hairs; 

 lice are horizontally flattened and are provided with strong 

 clasping organs to hold fast to hairs, and both of these examples 

 are wingless and have sacrificed much of the ordinary means of 

 locomotion. Entoparasites are usually provided with special- 

 ized hooks, barbs, suckers, etc., for purposes of attachment to 

 the alimentary canal or other organs, e. g. the botfly larvae, 

 and among the Helminths, the flukes (Trematoda), the tape- 

 worms (Cestoda), etc. Perhaps because of the ease in securing 

 food the sense organs are usually not strongly developed, the 

 eyes may be wanting or very simple. The mouthparts differ in 

 the several groups, depending on their special adaptation of 

 habit. It is interesting to note that the parasitic habit has re- 

 sulted in the development of structural similarity. This is 

 especially prominent in the clasping structures of the biting and 

 sucking lice, which belong systematically to two different orders, 

 namely, the Mallophaga and the Hemiptera. 



Origin of parasitism. Modern parasites are restricted 

 more or less completely to a particular host animal, which would 

 necessitate the deduction that the parasite must have developed 

 its habit after the existence of the host, and in consequence that 

 parasitism must be a recently acquired habit. This thought is 

 further expressed by the study of the life history of the parasite. 

 Invariably the earlier stages point to a free living existence. 

 Perhaps the ancestors of a given group of modern parasites were 

 attracted to the waste food, offal and exudations of certain 

 animals; the search for food may have become simplified; they 

 began living as messmates or commensalists, or as scavengers; 

 the association between the species may have become closer and 

 the eventual line of parasitism completed. This is also borne 



