INTRODUCTION. 



Paeasites and Parasitism. 



That many, if not most, animals have upon and 

 within them other and smaller animals and fungi — 

 parasites or ' guests,' as they are generally called — 

 is now a well-established fact. Regarding these 

 ' guests,' always unbidden, and their effects upon 

 their * hosts,' there is considerable universal ignor- 

 ance that Science is doing much to obliterate. The 

 scientist, also, is sometimes in the dark concerning 

 these parasitic forms ; but rapid strides have been 

 made in Parasitology during recent years. Many 

 complicated life-histories of parasitic forms have now 

 been unravelled, yet a vast field of research and 

 careful experiment is still open. At one time 

 parasites were thought to be part and parcel of 

 the host upon which they lived. This absurd notion 

 has long since been dispelled ; yet it is still most 

 difficult to make those who are not of a scientific 



