33'^ 



ARACHNOIDEA. 



CASE Professor Owen considers that it represents the lowest organised 

 form of the class Arachnidae and that it makes a transition from the 

 Anellids to the higher articulates. If an Acarid it comes nearer the 

 Sarcoptidse than any other, and we have thought the present as 

 proper a place as we could find for it. Still we own that it comes in 

 awkwardly, and as Dean Swift once said, if the judicious reader can 



assign it a fitter place, we do 

 here empower him to remove 

 it into any other corner he 

 pleases. The more important 

 points of affinity are the struc- 

 ture of the mouth which is 

 suctorial, the transversely stri- 

 ated skin, and its larval de- 

 velopment. The latter is 

 hexapod. 



Its habits are in some re- 

 spects similar to those of some 

 of the Sarcoptidse. It is not 

 a normal inhabitant of the 

 hair follicles or sebaceous 

 glands, but appears to make its way into them from without. 



Either the same or an allied species has been found in the 

 contents of the pustules of a mangy dog, when they occurred in 

 such abundance that thirty or forty were frequently seen in a 

 single drop of pus. It has not been discovered whether the 

 insect had anything to do with originating the mange, or had 

 merely taken advantage of its previous existence to establish 

 itself in the pustules, nor does it appear whether the instance 

 referred to was an isolated case or not (see Owen's Lectures on 

 Invert Comp. Anatomy, 2nd ed., p. 444). 



In 1872 Pennetier described a species which he named D. 

 caninus (Bull. Soc. Rouen), which is probably the same as the 

 above. 



Demodex folliculorum. 

 Copied from Owen's figure. 



Ditto in bulb and sebaceous 

 follicle of human hair. 



