108 ANIMAL PARASITES AND MESSMATES. 



free parasites, which take advantage of their host, 

 but with prudence and economy ; they ask from him 

 nothing but his blood, and sometimes render him im- 

 portant services. Many of these animals, both mess- 

 mates and parasites, have at present been only pro- 

 visionally classified, and cannot be definitely arranged 

 till more observations have been made. It is not always 

 so easy as it may be thought to determine exactly the 

 relations which certain animals have with each- other. 

 We must pry very narrowly before we can ascertain the 

 motives which act on this inferior order of beings. It 

 is among free parasites that we find those organisms 

 which are generally called vermin, and which seem the 

 more capable of injuring their neighbours since they can 

 the more easily escape detection. These creatures, 

 though they are called vermin, excite no more repug- 

 nance in the .mind of the naturalist than the other works 

 of creation ; and St. Augustine did not exclude them from 

 his thoughts when he exclaimed, "Magnus in magnis, 

 maximus in minimis." 



Leeches drink the blood of their victim, and when 

 they are gorged to the very lips, they fall off, taking a 

 siesta for weeks or months. Thus enjoying a repast 

 at very long intervals, it is useless for them to continue 

 longer at table ; and this is therefore another reason that 

 they should usually preserve their organs of locomotion, 

 that they may use them after their long period of diges- 

 tion. 



Like the annelids, they do not change their form, and 

 as they are only attached to their host for a short time, 

 naturalists have not thought fit to place them among 

 parasitical worms, or Helmintha. However, if we pass 



