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 Trombidions and certain Hydrachnse 

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

 autumnalis, known in France, at least in some locali- 

 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, acaridse which 

 Mons. Eobin refers to the genus Cheyletus, rather than 

 to that of the Acaropses. 



Fig. 25.— Cheyletus eruditus. 



