CHAPTER II. 

 CLASSIFICATION. 



THE metazoa causing diseases or injury to man are so 

 numerous, belong to so many families, and vary so greatly 

 in their habits, that a profound knowledge of zoology is 

 necessary for their satisfactory study. It is proposed in 

 this part to deal at length with the better-known parasitic 

 metazoa only, and with the diseases they cause. Other 

 branches of the subject are dealt with in less detail. 



Classification. In the division of living organisms into 

 kingdoms, orders, families, genera, species, the simplest 

 and most readily demonstrable of the recognized methods 

 of classification is the one adopted. Occasionally an 

 alternative plan is given. It must always be remembered 

 that no classification founded on a single character is 

 sound. For a complete classification, the life-history 

 from the egg to the adult form \vould no doubt be the 

 best basis. Unfortunately, this is known for but a small 

 proportion of the members of this sub-kingdom. 



It is not uncommon to find that some external character 

 in the adult is so frequently associated with certain types 

 of development that it may be used as the basis of classifi- 

 cation ; but by using this method errors will be made 

 occasionally. The larger the number of external characters 

 used as the basis the fewer will be these errors; but 

 classification so made must always be revised with any 

 advance in our knowledge of development and internal 

 structure. 



Living organisms have been divided into two kingdoms, 

 Animal and Vegetable. It is now usual to interpose a third 

 " intermediate " kingdom, the Protista, composed of uni- 

 cellular organisms, some of which, the Protozoa, in many 



