86 ANIMAL PARASITES AND MESSMATES. 



tryon, but which render to him in return precious 

 services. Thus those which Kve on the produce of the 

 secretions, or which clear the system of useless ma- 

 terials in exchange for the hospitality which they receive, 

 are not true parasites. These services are of a very 

 different character, and the duties which they some- 

 times perform for each other are in some respects ana- 

 logous to medical care. 



Every animal has its own parasites, which always 

 come from without. With some few exceptions, they 

 are introduced by means of food or drink. In order to 

 ascertain their origin, the naturalist must beforehand 

 study the food, that is to say, the prey or the plant 

 which furnishes the habitual nourishment of the host 

 which gives them shelter. 



A carnivorous animal, however, does not in general 

 content himself with a single kind of prey — one vora- 

 cious animal of this class devom-s all that comes in its 

 way ; another, more of an epicure than a glutton, chooses 

 with more discernment. But in the midst of this varied 

 kind of food there is always some species which forms 

 the staple of the usual bill of fare, and it is necessary to 

 find out what this is if we wish to ascertain the parent- 

 age and the metamorphoses of the parasite, since it is 

 that which conducts the parasite to its new destination. 

 The mouse is destined to the cat, and the rabbit to the 

 dog ; in the same manner, each one of the herbivora is 

 intended to be the prey of a carnivorous animal, if not 

 larger and stronger than itself, at least more cunning. 

 It is of great importance to discover the animal which 

 conducts the new-comer into his habitation. When we 

 know it, we have only to introduce into it the stranger 



