DISEASES WHICH ARE INDUCED BY PARTICULAR PARASITES 163 



of development clear a simple illustration will be sufficient. A certain 

 hydatid infesting the brain of the sheep, which causes the disease known 

 as gid, will, if given to a dog, be developed into a tape- worm possessing 

 certain well-defined characters which mark its origin. The mature segments 

 of this variety of tape-worm, when given to sheep or taken up by them 

 when feeding on jDastures, will produce the hydatid in the brain. 



Arthropedes. — In this division is included all animals with jointed 

 limbs, all kinds of insects which, either in their mature or larval form, 

 become parasitic permanently or temporarily to any of the higher animals. 



Horses are infested by the larval forms of certain flies (diptera), and 

 by some varieties of lice and acari or mange-mites, which produce diseases 

 of the skin of the animals which they infest. 



DISEASES WHICH AEE INDUCED BY PAETICULAR 



PARASITES 



EXTERNAL PARASITES OF THE HORSE 



Numerous parasites belonging both to the animal and plant kingdoms 

 take up their residence on or in the skin of the horse, and occasion consider- 

 able derangement. The common affection which is known as ringworm, 

 on account of the circular form which the eruption assumes, is due to a 

 fine hair- like fungus which is known as the TriclKyphyton tonsurans 

 or ringworm fungus. Other parasites of the skin are derived from the 

 animal kingdom, as lice, which, without producing any specific aftection, 

 cause a great deal of irritation and interfere with the animal's condition. 

 Acari or mites, which cause different forms of mange; larvge (maggots) of 

 certain insects which take up a temporary residence on the skin, and bore 

 their way into it, or are thence transferred by the animal itself, in the act of 

 licking or biting, to its own stomach, where they undergo a certain amount 

 of development prior to quitting their hold and being e.xpelled to complete 

 the necessary changes to form the perfect insect. Certain small thread- 

 worms, and a lower form of organic beings belonging to the protozoa, 

 known as psorosperms, also infest the skin. 



RINGWORM OF THE HORSE 



It may be observed at the commencement that ringworm in the horse 

 is a comparatively rare affection. In cattle it is constantly found in 

 young animals; very rarely, however, is it seen in an animal after the 

 age of one year. In the horse age does not appear to afford an}^ protection 

 from the disease. 



