Parasitology is the science which 

 treats of parasites. The word "parasite" 

 is derived from the Greek and means, 

 literally, "situated beside. " It was used 

 by the ancient Greeks originally for 

 people who ate beside or at the tables of 

 others, and referred both to sycophants 

 or hangers-on and to priests who col- 

 lected grain for their temples. While the 

 social meaning of the term has been par- 

 tially retained, it has been given a new 

 connotation by scientists. Parasites are 

 defined as organisms which live on or 

 within some other living organism, which 

 is known as the host. Parasitism is the 

 association of two such organisms. 



Parasites may be either animals or 

 plants--viruses, rickettsiae, spirochetes, 

 bacteria, yeasts, fungi, algae, mistletoe, 

 dodder, protozoa, helminths, arthropods, 

 molluscs, and even certain vertebrates 

 such as the cuckoo. The general prin- 

 ciples of parasitology apply to all. How- 

 ever, in this book we shall deal primarily 

 with animal parasites, leaving the plant 

 parasites to textbooks of microbiology. 



In our everyday thinking we consider 

 that animals can live in three main hab- 

 itats--land, fresh water and sea water. 

 A fourth habitat is the parasitic one, 

 which is quite different from the other 

 three. As a matter of fact, there are 

 quite a few different parasitic habitats, 

 each with its own characteristics. Para- 

 sites are found in the lumen of the intes- 

 tinal tract, on the outside of the body, in 

 the skin, in various tissues, in the blood 

 plasma, inside different types of cells, 

 and even inside cell nuclei. 



Parasites have arisen from free- 

 living animals. Some parasites closely 

 resemble their free-living relatives, but 

 others have undergone structural changes 

 which make them more suited to their 

 changed environment. Since these 

 changes have in many cases been the loss 

 of some power which their free-living 

 relatives possess, parasites have some- 



C/iapter 1 



IHTROdUCTm 



TO 

 PARASITOLOGY 



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