130 ANIMAL PATIASITES AND MESSMATES. 



At the commencement of the last century, a certain 

 physician attributed the cause of almost all diseases to 

 microscopical insects, and gave figures of ninety species 

 which were supposed to produce, in some cases small- 

 pox, in others rheumatism and gout, jaundice and whit- 

 lows. Almost all these figures represent imaginary 

 creatures. This opinion has reappeared in modern 

 times ; how many persons have been seen to smoke 

 camphor in order to preserve themselves from the 

 invasion of animalcules. I do not speak of the apparatus 

 which has been contrived in order to breathe nothing 

 but air which has been filtered and deprived of its living 

 germs. 



There are some of the articulata with four pairs of 

 feet, a kind of microscopic spiders which require to be 

 noticed here ; these are the numerous Acari which infest 

 many animals. Some of these wander on the surface of 

 the skin, others in galleries under the epidermis, and 

 many pass from one animal to another without changing 

 their form or mode of life. There is a considerable 

 number of them ; no class of the animal kingdom is free 

 from them, neither aquatic nor terrestrial animals, 

 neither vertebrates nor invertebrates. These parasites 

 belong for the most part to the same family, and cause 

 by their presence a disease which was for a long time 

 considered to be peculiar to the skin. 



An EngHsh naturahst, Mr. George Johnson, carefully 

 studied the parasitical and free acaridse of Berwickshire. 

 Mons. Ehlers has written a very interesting work, with 

 fine illustrations, on the acaridas of birds, published in 

 the ''Archives of Troschel." There is more than one 

 species which lives at the expense of man, and one of 



