The ectoparasitic arachnidans comprise a great variety of mites and ticks (_Acaridæ_ and _Ixodidæ_) more or less proper to man, and also a number of creatures which, though hardly to be reckoned as human parasites, are apt to transfer themselves from animals to man. Little more than an enumeration of the forms is possible here. The Common Scab or Itch insect (_Sarcoptes scabiei_) forms the type of a great variety of arachnids, generally spoken of as different species according to the host they dwell upon. Mégnin, however, in his beautiful memoir, quoted below, regards most of the forms of this genus (found on the horse, hog, sheep, dog, wolf, and other animals) as mere varieties. In man the female Acarus burrows beneath the skin, forming galleries or curved channels, in which she deposits her eggs. The irritation produced is not alone due to these excavations, but to the presence also of a poison which the mite discharges when feeding. The _Sarcoptes crustosæ_ of Fürstenberg, producing the Norway itch, is a variety, if, indeed, it can be called as much. Under the frightful name of _Dermatophagoides Schérémétewsky_ two parasites found on an herpetic patient have been described as new to science by M. Bogdanoff, but Mégnin points out that these Acari are only female and young male representatives of his _Chorioptes setiferus_ (var. _bovis_) respectively. In Newfoundland, Dr Le Roy de Méricourt discovered a singular species upon an officer who had come from Havannah (_Tyroglyphus Méricourti_, Laboulbène). It possesses enormous palpi, as in the genus _Chyletus_ to which Robin refers it. Another ectozoon, placed by Mégnin and others amongst the lowest types of Arachnida, is the well-known _Demodex folliculorum_. It is a gregarious species, a dozen or more examples often being present in a single dilated hair follicle. Though disfiguring to the human face it produces little harm. M. Gruby made it out to be a very common parasite, infesting forty out of sixty persons; but Mégnin, in his brochure (l. c. infra, p. 119), shows this statement to be an exaggeration. It infests on the average not more than one in ten persons. According to Gruby, moreover, a single follicle in the dog may contain 200 of these mites, another statement which Mégnin deems unreliable. The _Demodex_ of the dog is only a variety (fig. 53). Many other human Arachnids have been found, some of which appear to be genuine species, whilst others are accidental, so to speak. Of the former kind, perhaps we may reckon the two species discovered by Hessling (_Cœlognathus morsitans_ and _Entarsus cancriformis_). Of the latter sort, those found by Busk, Simon, and Bory de St Vincent may be cited. The mite found in Simon’s case was the _Dermanyssus avium_, which infests cage-birds. Probably it was the same species which Bory found on a lady; but in Busk’s negro sailor the mite may have been _D. gallinæ_ of the common fowl. Differing