[Illustration: FIG. 45.--Head of _Oxyuris vermicularis_. Highly magnified. After Busk.] _Oxyuris vermicularis_, Bremser.--Of all the parasites infesting the human body this is the one concerning which the medical practitioner is most frequently consulted, partly on account of its remarkable frequency in children, and more particularly on account of the difficulty often experienced in getting permanently rid of it. The _Oxyuris vermicularis_ is by no means confined to young persons, seeing that adults are infested even to old age. It is familiarly known as the threadworm or seatworm. The male measures about 1/6″, and the female from 1/3″ to 1/2″ in length. The female possesses a long capillary tail, which terminates in a three-pointed end. The extremity is said to act as a kind of holdfast. The tail of the male is obtusely pointed. In both sexes the body presents a more or less fusiform shape, the anterior end being narrowed to form a somewhat abruptly-truncated head, which is often rendered very conspicuous by a bulging of the transparent integument surrounding the mouth. This presents in profile the aspect of winged appendages (fig. 45). The oral opening is tripapillated, leading into a triangular œsophagus. The integument is transversely striated, and of a silvery-white appearance. The spicule is simple, single, and very minute. The eggs are oblong and unsymmetrical. They measure about 1/900″ from pole to pole, and 1/1400″ transversely. [Illustration: FIG. 46.--Section of a female _Oxyuris vermicularis_, magnified 220 diameters (after Busk); and also several free eggs (original). _a_, With an imperfectly formed embryo; _b_, _c_, _d_, with three tadpole-shaped embryos, magnified 450 diameters.]