thp: art of parasitism 



223 



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Longitudinal section showing mistletoe invading 

 the tissues of its host. 



Permanent Parasites. Comparatively few organisms belong in 

 this category. Some of the flukes with a continuous life cycle like 

 the marine Epidella melleni, or the gill fluke, Ancyrocephalus, pass 

 their entire cycle upon the 

 same host, adding their 

 progeny to the same ani- 

 mal and so on ad infinitum. 

 Similarly, the female head 

 louse that cements her 

 eggs, or "nits," to human 

 hair from which newly 

 hatched lice appear within 

 six to ten days is another 

 example of a permanent 

 parasite. The new addi- 

 tions to the community of 

 head lice must soon feed 

 upon the roots of the host's 

 hair or else they will die. 



The parasitical mistletoe is practically permanent in habit, since 

 it not only taps the life-giving fluids of its host but also lives for 

 many years upon the same tree. 



Internal Parasites 



The food cycle plays a vital role in the dispersal of all internal 

 animal parasites. It frequently happens that animals which suck 

 the juices of plants or the blood of other animals play an important 

 part as an intermediate host. It should be borne in mind that when 

 carnivores are included in the chain of parasitism, the cycle tends 

 toward greater complexity. A few examples will serve to illustrate 

 this point. 



Parasites Requiring One Host. The adult hookworm, Necator 

 americanus, lives in the small intestine of man, where the adult female 

 is attached to the walls of the intestine and produces great numbers 

 of eggs which are eliminated from the digestive tract in the early 

 developmental stages. Under proper conditions of soil, temperature, 

 and moisture, development of the larvae proceeds rapidly, so that 

 hatching may take place within 24 hours. The small larval form is 

 only about 0.25 mm. in length, but by the end of the third day it has 

 nearly doubled in length and soon molts twice, then being in the 



