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XL] is not found on the grassy plains of the highlands of British East Africa. Towns and cultivated areas are free from it. This peculiar limitation of its distribution would indicate an intermediary restricted to similar stations, certain Tabanidæ, for instance, or certain species of sylvan mosquitoes. In order to find the intermediary host of F. perstans, Low made numerous experiments with many different species of mosquitoes (Culex fatigans, C. atratus, C. viridus, C. Inteolateralis, C. quasigelidus; Anopheles argyrotar sis, A. costalis, A. funestus; Stethomyia nimbus; Janthinosoma musica; Mansonia africana; Uranotœnia cœruleocephala; Tœniorhynchus fuscopennatus) and other blood-sucking insects (Pulex irritans, Sarcopsyila penetrans, Pediculus capitis, and P. vestimentorum). These insects were either caught in the huts of infected persons or were reared from the larva and then fed on infected persons. After a certain time they were dissected, but, with one exception, the results were negative. In one isolated instance two developmental forms were seen between the thoracic muscles of a Tœniorhynchus fuscopennatus reared from the larva and fed on infected persons. Fülleborn has found similar developmental forms once in An. maculipennis. Christy suggests that the true intermediary host of F. perstans is Ornithodoros moubata, a tick of the sub-family Argasinœ. He gives no evidence in support of this view, which is contradicted by the facts of geographical and topographical distribution. Pathology.— So far as known, F. perstans has no great pathological importance ; the presence of the adult parasites in the mesentery appears to cause little harm to the host.

In examining blood sent me by Dr. Newsam from natives of St. Vincent, West Indies, I found this microfilaria in several individuals— in 10 out of 152 examined. It resembled mf. bancrofti and mf. loa so far as shape is concerned, but differed from them in size, its average measurements being, according to Low, 0.2 mm. by 0.005 mm. It is sharp-tailed, has no sheath, and observes no periodicity, being present in the peripheral circulation both by day and by night. Its movements are very active; it can shorten or elongate itself, and not only wriggle about very actively, but travel from place to place on the fresh blood slide. Nothing is known of its life-history or pathological bearings. I have met with the same parasite in the blood of natives of St. Lucia, W.I., an observation confirmed by Galgey, Low, and St. George Gray. Low has also found it in the blood of natives of Dominica and Trinidad. Usually some eight or ten parasites are found in a preparation of ordinary dimensions; occasionally instances of high degrees of infection are met with, in which hundreds of microfilariæ can be counted on every slide. Its topographical distribution is singularly circumscribed, even in the endemic districts.