CHAPTER XXXII 



PARASITISM 



ILLUSTRATED BY THE FLATWORMS 



Parasitism is a very common association of animals of different species, 

 characterized in a general way by the fact that one individual, known as 

 the parasite, lives at the expense of another, called the host, but does not 

 devour it. Some parasites infect only one host, but in other cases they 

 pass successive periods of their lives in different hosts. In the latter 

 case the host in which they reach the adult condition is known as the 

 final host; and those in which they live during their larval development, 

 intermediate hosts. 



200. Structure of Parasites. — The parasitic flatworms illustrate 

 several of the salient features of internal parasitism. The loss of 

 certain organs by the parasite has been noted. The organs markedly 

 affected are those of the digestive and nervous systems. On the other 

 hand, other organs become more highly developed and new structures 

 appear. Such structures are hooks and suckers, which serve for more 

 effective attachment and result in more perfect adaptation of the organ- 

 ism to the conditions of parasitic life. The epidermis is modified to 

 resist the digestive juices of the host. Of the organs which show increased 

 development the reproductive organs are the most prominent. There 

 is so much uncertainty attached to the conditions of life that if it were 

 not for the production of an enormous number of eggs, these parasites 

 would cease to exist. At several points in the life history of the liver 

 fluke and at two places in the life history of the tapeworm chance deter- 

 mines whether the life history is to come to an end or to continue. It is 

 stated that a single liver fluke may give rise to as many as 500,000 eggs, 

 and estimates of the number of eggs produced by a single tapeworm 

 reach 3,400,000,000 (Penfold, Penfold, & Phillips, 1937). 



201. Sheep Liver Fluke. — The development of the sheep liver fluke 

 {Fasciola hepatica Linnaeus) is usually taken as a type of that of the 

 trematodes. This fluke is relatively large, being about an inch long. 

 The most frequent final hosts are sheep and cattle, but it may be found 

 in any one of a number of smaller mammals and rarely in man. The 

 parasite may be lodged in other tissues than the liver. In man it has 

 been known to live four years. This fluke is found in various parts of 

 the world wherever sheep are raised, and different species of snails serve 

 as intermediate hosts in different regions. 



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