SYNOPSIS OF ANIMAL PARASITOLOGY 



INTRODUCTION. 



Parasitism is a condition of widespread occurrence in both 

 plant and animal kingdoms, though much more characteristic of 

 certain groups than of others. Its basis is to be sought chiefly in 

 a nutritive relation in which one organism, the parasite, obtains 

 its food-supply directly from the living tissues of another organism 

 which in this way becomes the host. In this condition it is purely 

 a matter of the degree of parasitism whether the infesting organ- 

 ism, naturally a smaller species, occupies a superficial position on 

 the body of the host, as an ectoparasite, or as an endoparasite, 

 penetrates into its interior. 



This parasitic association is one of the results of competition 

 among organisms, and is accompanied by structural changes 

 analogous to those taking place in other directions in free-living 

 organisms by adaptation and specialization. All organisms tend 

 to utilize the available resources of food-supply, but a fundamental 

 difference exists between those possessed of carbohydrate-building 

 power, after the plan of the green plants, and those which, lacking 

 this property, are dependent for their supply of organic food on 

 preformed material. It is the latter group, comprising all animals 

 and many plants, to which the developments of parasitism especi- 

 ally apply; since while the majority of species, the free-living 

 animals, so called, as well as the saprophytes among plants, 

 obtain their food-supply from dead organic material, whatever its 

 original source, many others as parasites form a direct association 

 with the tissues of a living host. The utilization of this source of 

 food-supply and the adaptations produced must accordingly be 

 interpreted in the same manner as for free-living organisms. The 

 evolutionary development of all organisms has its beginnings in 

 free-living habit, and accordingly the classification of organisms 

 into natural groups is based mainly on the characters of free-living 

 forms. All parasites belong to these same major groups and this fact 

 facilitates their classification, especially when degenerative changes, 

 which conmonly take place in portions of their organization, have 

 obscured their relationships. Many natural groups contain 

 parasitic members, but the occurrence of greater proportions of 

 parasitic forms in such groups as the Platyhelminthes and Nemat- 



