SOME REACTIONS AMONG ANIMALS 155 



d. Parasitism 



Note. Many other animals have taken to a thievish exist- 

 ence as unbidden guests in or on the bodies of other animals. 

 The "thieves" are called parasites and the animals preyed 

 upon are known as hosts. In this relation the parasite some- 

 times gains all the necessaries of life, while the host gains 

 nothing, and may even lose its life. 



i. Grades of Parasitism 



Observations. 1. Fleas on cats and dogs are known as 

 temporary external parasites. Do they necessarily remain on 

 one animal ? 



2. Chicken or bird lice are known as permanent external 

 parasites, as they remain on one host until death of parasite 

 or host. 



3. Many crustaceans, insects, and worms live within the 

 bodies of their hosts. These are known as internal parasites. 

 Many times two hosts are necessary. The tapeworm lives in 

 man until eggs are set free, then the eggs are eaten with food 

 by cattle or pigs, then the young worms eaten in underdone 

 beef or pork and thus taken into the body of man again. 

 Here man is the host and the cow or pig is known as the inter- 

 mediate host. 



2. Probable Causes and Advantages of Parasitism. 



Observations. 1. Show how the females of chicken lice or 

 fleas may be better off than they would be if free-living. 



2. Show how it may be easier for parasites to get their 

 food supply than if they were free-living. 



Conclusion. Tell as many of the causes and therefore ad- 

 vantages of parasitism as you can. 



3. Some Parasitic Worms 



Observations. 1. Examine a tapeworm, or study a Figure of 

 one. Note its segmented shape. How long do these worms 

 grow? 



