8 The Sheep- Scab. 



What is the Scab? 



The scab is a contagious skin disease of sheep, 

 due to the presence of animal parasites belong- 

 ing to the class arachnida (spiders) ; order 

 acarida (mites); sub-order crawling mites; fam- 

 ily sarcoptes, of which there are three genera: 

 — the sarcoptes, that burrow in the skin; the 

 dermatodectes, that simply bite and hold on to 

 the skin; and the symbiotes, that live together 

 and pierce no farther than the epidermis, in 

 search of food. 



The variety of acarus we have to consider is 

 the sarcoptes ovis (that which burrows in the 

 skin of the sheep). It is a spider-like mite 

 from one one-hundredth to one-sixtieth of an 

 inch in length. They have no tracheae or other 

 respiratory organs, as yet discovered by investi- 

 gating naturalists, hence respiration is supposed 

 to be cutaneous. 



There are different species of the genus 

 acarus, of which the one under consideration is 

 peculiar to the sheep, for although the disease of 



