CHAPTER V. 



ANATOMY. 



External Anatomy. 



The Cuticle. The integument of the Tyroglyphidse 

 is usually soft and but slightly chitinised. There are, 

 however, exceptions to this rule ; thus the cuticle of 

 Ghortoglyphus is much harder and more highly chiti- 

 nised than that of most other genera. In Hericia 

 numerous chitinous plates are sunk in the dorsal cuticle, 

 and often serve for the attachment of dorso-ventral and 

 levator coxse muscles. The skin is often highly polished, 

 as in most species of Tyroglypkus, Rhizoglyphw, etc. 

 In other genera, such as Glycyphagus, it is dull, even 

 rough, and in some cases like shagreen ; in Hericia and 

 some species of Glycyphagus, such as G. platygaster, 

 the dorsal cuticle is covered with small stout points. 

 It very rarely presents the regularly spaced pores which 

 are so commonly found in families of Acarina, which 

 have a denser and more highly chitinised cuticle, such 

 as the Oribatidse. 



Probably in most, if not all, Acarina the integument 

 may be considered to consist of three layers or tunics ; 

 I have usually called these the epiostracum, the ecto- 

 stracum, and the endostracum, following the names 

 which Huxley adopted in ' The Crayfish.' Some writers 

 consider that the cuticle consists of two layers only, 

 but that in many cases one of these is divided into two ; 

 it is not very important which view is taken, and it 

 probably depends chiefly upon what group of Acari the 

 particular writer has paid most attention to. In the 



