4 SOME MINUTE ANIMAL PARASITES 



dependent lives, swimming freely in water or moving 

 slowly over the surface of plants or of other animals, 

 and obtaining food from the surroundings in which 

 they live. Others have become sedentary, and while 

 independent of their hosts so far as their food-supply 

 is concerned, derive the immense advantage of trans- 

 port from place to place by attaching themselves to 

 the exterior of some more active animal. With the 

 sedentary habit, the necessity for development of 

 means of locomotion has come to an end, and as a 

 result, the Protozoon degenerates and its structure 

 then appears simpler than that of its more active 

 relations. The final degradation of the protozoal 

 world is reached when the habitat of the Protozoa 

 of economic importance is described. These tiny 

 organisms are so degenerate that they have become 

 parasites, deriving their nourishment from the body 

 of the living animal to which they have attached 

 themselves, and are incapable of prolonged existence 

 apart from their host except in specialized form. 



The ways by which parasitic Protozoa gain access 

 to their hosts are of considerable interest. A large 

 number of these organisms, towards the end of their 

 life-cycle as individuals, become small bodies which 

 are rounded or oval, have great powers of endurance, 

 and are thoroughly adapted for life outside the body 

 of the host. These forms may be termed resting or 

 resistant stages. Many of the parasites possessing 

 resistant forms are spread from animal to animal 

 in a very simple way namely, by the new host 

 swallowing food or drink contaminated by the 

 resting stages of the parasite. These resting bodies 



