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XII] months. Preventive inoculations have been successfully practised in rats, mice, and monkeys. Infected animals can be promptly cured by the injection of hyperimmunized serum, and relapses can be prevented by the same means. Although Todd has not been so successful in similar experiments, it seems probable that we have in these observations a basis for the prevention and cure of relapsing fever in man.

Prevention.— The fact that the spirochæte is conveyed to man by bug, or louse, or tick bite indicates that personal and domestic cleanliness and the avoidance of people and places infested with such vermin must form the basis of successful prophylaxis. Especially to be avoided in Africa are the resting-places of caravans and travellers, and the huts of natives. The mosquito net, a bed well off the ground, and a night-light are indispensable in that country, where the nocturnal habits of Ornithodoros moubata render the hours of sleep especially dangerous.

Recent developments in human and veterinary pathology have shown that ticks play an important part in the transmission of disease. They are widely distributed, almost every animal either having species special to itself or being liable to attack by species of a wider zoological range. They belong to the order of the Acarina, of which they are by far the largest forms. They are always visible to the naked eye, and the females are almost invariably larger than the males. In some species the ovigerous females, when gorged with blood, may reach a length of nearly half an inch. As a rule they are temporary parasites, but some live in a quasi-permanent manner on the body of their host; and occasionally a few, as the sheep tick, Ixodes reduvius (Plate VI., 2), may even burrow beneath the skin. They differ from insects in possessing four pairs of legs, and in having the three regions of the body— head, thorax, and abdomen— fused into one unarticulated mass. This latter feature also distinguishes them from the spiders, in which the abdomen is clearly distinct from the cephalothorax. After impregnation the female tick attaches herself to her host. Becoming enormously distended with its blood, she drops off and secretes herself in some convenient hiding-place where she deposits her eggs, which are small, yellowish, roelike grains, amounting in some cases to thousands. Oviposition begins from two to ten days after the host has been quitted, and goes on for several days. In due course (two or three weeks under favourable conditions) the eggs are hatched. The larvæ