stroyed before this extreme stage is reached. This complaint is chiefly seen in great towns. S. mutans (Fig. 13) is parasitic on birds and 
Fig. 13.—Sarcoptes mutans—Male, underside. domestic fowls, appearing on the feet, on the comb, and about the beak, gradually spreading until the whole body is involved. Other species have been found on other domesticated and wild animals, while a large genus—the louse-mites, Myobia—infest the smaller mammals, as rats, mice, cats, etc.
The Demodex folliculorum is a minute animal, 1100 to 150 of an inch in length, that lives in sebaceous sacs and hair follicles of the human skin, particularly about the nose (Fig. 14). Its habits are in some respects similar to those of the itch-mites. It is not a normal inhabitant of the follicles or glands, but appears to enter them from without. If pressure be made upon any one of the sebaceous 
Fig. 14.—Demodex
folliculorum. follicles that are enlarged and whitish, with a terminal black spot, matter will be forced out, consisting mostly of accumulated sebaceous secretions, in which the parasite, if present, will be imbedded together with its eggs and young. The secretion may be softened with oil, so that these may be separated and the animal removed with a pointed brush. They do not seem to be present in all persons, occurring in two or three cases out of ten, and seeming to prefer thick, greasy skins. They are entirely harmless, and their presence is no indication of disease. The family roll of the acari closes with the gall-mites, Phytoptidæ, a curious and little-known group that causes the abnormal growths known as galls on the leaves and other parts of various plants. Any extended reference to them here would be out of place. The whole of the order Acarina afford interesting objects of investigation, being in close relations to man, and yet very imperfectly known. Some of their forms are always attainable, and an examination of them under the lower powers of the microscope is one of the most amusing objects of study that can be presented to the young.
