Numerous parasites belonging both to the animal and plant kingdoms take up their residence on or in the skin of the horse, and occasion considerable 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 Trichophyton tonsurans or ringworm fungus. Other parasites of the skin are derived from the animal kingdom, as lice, which, without producing any specific affection, cause a great deal of irritation and interfere with the animal's condition. Acari or mites, which cause different forms of mange; larvae (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 expelled to complete the necessary changes to form the perfect insect. Certain small threadworms, and a lower form of organic beings belonging to the protozoa, known as psorosperms, also infest the skin.