PAEASITES AND PAEASITISMUS. 199 



plants may be beneficial to the organisms in whicb they 

 live. On the other hand, in parasites we often note that 

 nature finds a means of removing a surplus animal popula- 

 tion. Thus, in some years, sheep, cattle, and horses, fall 

 victims to the " worms '^ in large numbers, as seen in the 

 diseases rot, husk, &c. The obtaining of parasites also 

 depends on the nature of the diet; vegetable food cannot 

 prove the direct bearer of the majority of parasites, but 

 many can be taken in with meat. 



It must be distinctly understood that it is quite the 

 exception for a few parasites to cause serious mischief, 

 since they generally give rise to disorders only when they 

 are present. in large numbers. The manner in which these 

 few act is either by producing functional nervous disease or 

 destroying the walls of the organ in which they are situated. 

 These actions are rarely, if ever, seen in the ox. Some 

 parasites do not live upon the structures of the host, but 

 simply inside or upon other animals not being dependent 

 directly upon them for nutriment. Such are fellow- 

 hoarders J a barnacle on a whale is an example of this 

 relationship. From this the interests of the two indi- 

 viduals tend more and more to clash. The fluke resides 

 in the liver, and consumes the bile which would be useful 

 to its host ; other forms prey on mucus, some on the 

 contents of the alimentary canal, and some, finally, are 

 blood-suckers, and thus prove to the highest degree 

 exhausting. 



Though these parasites are intimately associated with 

 the pathological conditions to which they give rise, and 

 often dependent on them at certain phases of their life, 

 the processes prove more or less troublesome to the host ; 

 and if the invaders- have been numerous the conditions to 

 which, by their combined efforts, they give rise may lead 

 to severe disorder and even death. Thus, the ill effects 

 of parasitic organisms are, in many cases, directly related 

 to the number of them present. 



Yet it must be remarked that except where essential to 

 tlie well-being of the parasite, the disorder which it 

 causes is not nearly so severe as that which would result 



