Filaria medinensis, the thread or Guinea-worm, of about the thickness of packthread, whitish, from half a foot to several feet long, at the broader end obtunded, terminating behind in a pointed curve. Peculiar to tropics of the Old World, but especially to Guinea; inhabits the subcutaneous areolar tissue, especially of the lower extremities, but occasionally also of the scrotum, the trunk, and the throat. Having spent its earlier period out of the body, it burrows beneath the skin, where it tarries in the areolar tissue for a considerable time (several months), after which it again perforates the skin from within, in order to disburden itself of its offspring, or in order, it may be, to migrate for this purpose altogether. These proceedings are attended with inflammation and ulceration, and great caution is recommended, in any attempt to extract the worm, to avoid tearing it; either the elapsing brood or other contents of the worm, having an erosive property which tends to aggravate the said processes. It would seem that, as yet, none but females have been observed. Accordingly these must have introduced themselves in the impregnated state.

Trichocephalus dispar, the hairhead or whip-worm; filiform; the anterior part hair-like, the posterior part considerably thicker; from one and a half to two inches long; of distinct sexes. The male is, at its posterior part, spirally convoluted, and its penis contained in an elongated, funnel-shaped, violet-colored sheath. The posterior part of the female is not spiral.

It infests the caecum, singly, and also frequently in multitudes [especially, it is true, in the dead bodies of persons who have died of protracted typhus or similar diseases], without occasioning any extraordinary symptoms. The females are loaded with ova, which are, however, not developed in this locality.

Ascaris lumbricoides, the cylinder worm, a widely disseminated intestinal worm, from five to twelve inches in length, cylinder-shaped, tapering towards both extremities, especially towards the anterior; having four longitudinal striae, two of which are more strongly pronounced; densely marked with transverse striae: semi-diaphanous, so that the intestinal canal and the organs of reproduction are transpicuous. The head, divided from the body by an annular groove, displays three little nodules, or rather valves, which encircle the mouth.. The caudal extremity, especially in the male, is incurvate. Sexes distinct, the male being smaller and narrower, and having at the caudal extremity a thin, capillary, sometimes double penis. The female is larger, and exhibits at its upper third a fissure from six to eight millimetres long, as the orifice to the organs of generation, which contain ovaries and oviducts of enormous length.

It infests the ileum, often in extraordinary number, in groups and conglomerate masses. A brood is never met with; the ova, therefore, become hatched extraneously to the human body, to remigrate thither afterwards, as the living brood. It gives rise to the well-known worm symptoms. The perforation of the intestine, however [migration extraneous to the intestinal membranes], and its sequelae are, to say the least, extremely rare.

Oxyuris vermicularis (the Ascaris vermicularis of Rudolphi), the hook-tail, maw-worm; - a little, thin, white worm. Sexes distinct. The male very rare and small, from one to one and a-half millimetres long, with spiral convoluted tail: annulate; with a tail terminating in a fine transparent point. The head of either displays a transparent swelling, which under the microscope appears as a wing-like membrane.

It inhabits the colon and especially the rectum, occasioning both here and in the vagina, into which it creeps, an intolerable itching. As it is never accompanied by a brood, it probably migrates as the impregnated female.