46 BRITISH TYBOGLYPHIDJ1. 



Oribatidse the three layers are usually perfectly distinct, 

 and the epiostracum often rubs off leaving the other 

 two layers intact ; an almost similar thing often occurs 

 in the Hydrachnidae, or in such of them as have chitinous 

 plates embedded in the cuticle, e. g. Thyas, Hydro- 

 droma, etc. ; the epiostracum often persists above the 

 plates when it has rubbed off elsewhere. In fidftlla 

 and Trombidium it is usually difficult, if not impossible, 

 to detect more than two layers. I have discussed the 

 question of the two or three layers and their nomencla- 

 ture in 'British Oribatidge ' (p. Ill), and do not think 

 it necessary to repeat it here. Nalepa considers that 

 the integument of the Tyroglyphidse consists of two 

 layers, which he calls the " chitindecke " and the 

 " matrix ; " he says that the former is usually a thin, 

 structureless, and tolerably stretchable tunic ; but that 

 in some cases it becomes greatly thickened by the 

 deposition of lime-salts, usually accompanied by pig- 

 ment, and then becomes hard and brittle and generally 

 loses its character of a layer. There is no doubt that 

 the commonest character of the two other layers of the 

 cuticle in adult Tyroglyphidse is what Nalepa describes ; 

 namely, that the distinction between epiostracum and 

 endostracum is lost, and that the two form a single 

 flexible and stretchable tunic in which little, if any, 

 structure can be detected ; but there are species in 

 parts at least of whose bodies the three layers of the 

 integument may be quite plainly seen. Thus in Grlycy- 

 phagus platygaster (PL C, fig. 4) the epiostracum of the 

 lateral and dorsal portions of the abdomen is a thin, 

 structureless tunic, but is roughened on its exterior 

 surface by thickly-set almost triangular points; the 

 ectostracum is a much thicker tunic, and is easily dis- 

 tinguishable from the other layers. 



The endostracum, matrix, or hypoderm is the living 

 tunic. Nalepa says that in Odrpoglyphus it does not 

 consist of a distinct cellular layer, but is a loose net- 

 work of anastomosing cells whose nuclei are rare and 

 are surrounded with scanty plasma; he remarks that 



