PARASITES FREE DURING THEIR WHOLE LIFE. 



131 



them produces a disease known in every country and at 

 all times under the name of 

 the itch; until 1830 its true 

 nature was still unknown. It 

 is not an affection of the skin, 

 as was thought, but merely 

 the result of the presence of 

 these animalcules. The di- 

 rector of the special Hospital 

 for Skin Diseases at Paris was 

 so fully convinced that the 

 acaridse are not the cause of 

 the itch, that he offered a 

 prize to any one who could 

 render these insects visible. 



Sarcoptes scabiei, or male 

 acarus of the itch ; the lower 



Fig. 21 

 acari 

 surface 



A student of medicine, a 



Fig. 22.— Sarcoptes scabiei, female ; the upper surface. 



