10 



Relation to free-living forms — Readily seen in ectoparasites and 

 in certain endoparasites. Clearly shown by the developmental 

 history of others. All evidence shows that, — 



Parasitism is an adaptive phenomenon, originating independently 

 in widely different groups. 



We shall therefore briefly outline the major groupings of animals, 

 before considering more fully the parasitic forms. 



THE ZOOLOGICAL RELATIONS OF PARASITES 



INTRODUCTION 



Zoological classification — Necessity for classification — Great 

 variety of animals. Some estimates place number of species as 

 high as 10,000,000. Popular attempts at classification based on 

 superficial characters. 



Bearing of the theory of evolution on zoological classification — Aim 

 of naturalists to work out a natural classification, one that will 

 express blood relationship. 



Names and sequences of the different kinds of groups — Animal 

 kingdom, phyla or sub-kingdoms, classes, orders, families, genera, 

 species, varieties, individuals. 



Characteristics of these groups — Definition of species — "In 

 general terms, a species is a collection of individuals which resemble 

 each other as closely as the offsprings of a single parent. As a rule 

 the pairing of sexes of different species will not be fertile." — Com- 

 stock. 



Zoological Nomenclature — Necessity for an international code 

 of nomenclature — (See, Stiles, '05, which should be obtained by 

 students planning to specialize along zoological lines). Same 

 species often named independently by two or more workers. Especi- 

 ally true of parasites, which have been often named by physicians 

 or others with little zoological training. One species of tapeworm 

 has received 110 different names. 



The more important rules of zoological nomenclature — 

 1. The name of a species must consist of two words, and only 

 two, one indicating the genus to which the species belongs and the 

 second used to designate that particular species. 



