(4) Trichina (Trichinella) Spiralis.--This is a very small worm, not exceeding 3 mm. in length when fully developed. Infection in man occurs from ingestion of insufficiently cooked pork, which contains encysted embryos. These reach maturity in the small intestine. The female produces great numbers of young, which {256} migrate to the voluntary muscles, chiefly near the tendinous extremities, and there become encysted. Trichiniasis is generally accompanied by a marked eosinophilia. The diagnosis is made by teasing out upon a slide a bit of muscle, obtained preferably from the outer head of the gastrocnemius, the insertion of the deltoid, or the lower portion of the biceps. The coiled embryos can easily be seen with a two-thirds objective (Fig. 108). [Illustration: FIG. 109.--Uncinaria duodenalis: _a_, Male (natural size); _b_, female (natural size); _c_, male (enlarged); _d_, female (enlarged); _e_, head; _f_, _f_, _f_, eggs (after v. Jaksch).] (5) Uncinaria.--The two varieties of this worm, _Uncinaria duodenalis_ and _Uncinaria Americana_, are among the more harmful of the animal parasites. They inhabit the small intestine, usually in great numbers, and commonly produce a severe and often fatal anemia. Infection is common in subtropical regions, notably in Egypt, in some European countries, and, especially, in Porto Rico and the West Indies, where about 90 per cent. of the rural population is infected. It is much more {257} common in the United States than was formerly supposed. The adult worms are seldom found in the feces, but may appear after a dose of thymol followed by a brisk purge. They resemble _Oxyuris vermicularis_ to the naked eye. Ova are usually present in enormous numbers. Those of _Uncinaria duodenalis_ measure about 30 by 50 µ; of _Uncinaria Americana_, somewhat more. They have a thin, smooth, transparent shell, and their protoplasm is divided into 2, 4, 8, or more rounded segments (Fig. 110). [Illustration: FIG. 110.--Four eggs of the New World hook-worm (Uncinaria Americana), in the one-, two-, and four-cell stages. The egg showing three cells is a lateral view of a four-cell stage. Greatly enlarged (after Stiles).] (6) Strongyloides Intestinalis.--Infection with this worm is by no means so rare in this country as the few clinical reports would indicate. It is very common in subtropical countries, notably in Italy and in southern China. It seems probable that the parasite is the cause {258} of "Cochin China diarrhea," although some authorities regard it as harmless.