as affectinfi Domesticated Animals. 71 



its abdomen from l-88th to l-227tli, while in both extremes 

 the width of the thorax is the same, viz. l-555th of an inch. 



" The animal is divisible into a head, thorax, and abdomen, the whole of 

 tliese imrts being well and distinctly marked. The head represents in form a 

 truncated cone flattened from above downwards, and directed obliquely down- 

 wards from the anterior part of the trunk. It is composed of 2 large lateral 

 organs, termed by Simon, ' maxillary palpi,' and of an intermediate triangular 

 organ." . , . "The thorax, which is the broadest and tliickest part of the 

 animal, and somewhat tun-shaped, is flattened on its nnder surface. It is 

 composed of four broad segments, wliich are free, and joined by a connecting 

 membrane on the dorsum and sides of the creature, but are continuous inte- 

 riorly with the broad and strong plastron which covers the whole inferior 

 surface of the thorax. The segments are somewhat convex in their antero- 

 lX)sterior diameter, particularly at the upper part, so that the outline of the 

 chest in this situation has the a])pearauce of being slightly fluted." . . . "The 

 segmented structure of the thorax permits of a certain degree of movement iu 

 this part of the creature. The legs, which are eight in number, are connected 

 with the sides of the plasti-on ; each segment of the thorax sustaining one pair 

 of these organs. They are conical in figure, the base of the cone being broad, 

 and its apex obtusely truncated, and furnished with 3 finger-like claws. P]ach 

 leg is composed of 3 segments." ..." The legs are all of the same size." . . . 



" The aidoinen is somewhat variable in point of length. It is flattened on 

 its under surface, and convex above, and tapers gradually from its base to its 

 extremity, where it terminates iu a rounded ])oint. It is composed of a series 

 of extremely narrow annular segments, which overlap each other from before 

 backwards." ..." The annulated structure of the abdomen, which is here 

 described, permits it to move with considerable freedom and to curve in any 

 direction." 



Mr. Wilson closes his elaborate description of the anatomy of 

 the Acarus by remarking, " In the abdomen I have traced the 

 outline of an alimentary canal, and have seen it terminate by an 

 infundibulifonn extremity at the anus. The transparent cell- 

 like organs seen in the abdomen of the perfect animal I regard 

 as dilatations or convolutions of the alimentary canal ; and a 

 dark-brownish mass in the commencement of the abdomen I 

 consider to be the liver. I have been unable to discover any 

 sexual differences in the numerous examples which I have exa- 

 mined." * 



The existence of these parasites in connexion with a dis- 

 eased condition of the skin seems not to be admitted in the 

 case of man ; but it is a vexed question among comparative 

 pathologists as to whether they do not give rise to a diseased 

 state of the skin of the dog. It is well known that the dog is 

 remarkably prone to skin diseases, several of which have but few 

 features in common ; and our experience — as has been shown — 

 would go to prove that one at least of these affections may depend 

 upon these parasites. Gruby has even stated that the Acari of 

 man will induce disease of the skin if transferred to the dog ; 

 and that having once succeeded in so transferring them, he 



* Wilson on ' Skin Diseases,' 4th edit., p. 729, et teq. 



