PARASITES FREE DURING THEIR WHOLE LIFE. 137 



North Pole, on board the Polaris, has seen the species 

 of the anodonts crossed with those of the Uniones. 



There are also Arachnida which are parasitical only 

 while young, as the Tromhidions and certain Hydrachnse 

 (Fig. 24) which frequent aquatic animals. The Leptus 

 autumnaUs, known in France, at least in some locaU- 

 ties, by the name of Rouget, ^is an acarian which 

 throws itself upon 

 man, and especially 

 attaches itself to the 

 roots of the hair : 

 fortunately, it is only 

 found in the country 

 districts. The Acarus 



{Cheyletus) eruditus 



(Fig. 25) lives in books 



and collections, as well 



as on fruits and all 



kinds of bodies more 



or less damp, left in 



dark places ; it has 



been studied by Van 



Der Hoeven. Mons. 



Leroy de Mericourt 



found in pus, which 



was running from the ear of a sailor, acaridae which 



Mons. Kobin refers to the genus Cheyletus, rather than 



to that of the Acaropses, 



Fig. 25.— Cheyletus eruditus. 



