The adult worm, which reproduces by parthenogenesis, is about 2 mm. long. It inhabits the upper portion of the small intestine, but neither it nor the ova appear in the stool unless an active diarrhea exists. Ordinarily the eggs hatch in the intestine, and when infection is severe, embryos can be found in the feces in large numbers. These are the "rhabditiform embryos," which measure about 0.40 by 0.02 mm. They are actively motile, and are best found by making a small depression in the fecal mass, filling it with water, and standing in a warm place (preferably an incubator) for twelve to twenty-four hours. The embryos will collect in the water, and can be easily found with a two-thirds objective. [Illustration: FIG. 111.--Strongyloides intestinalis: A, Mature female; B, rhabditiform larva; C, filariform larva (after Braun).] Outside the body the rhabditiform embryos develop into a free-living, sexually differentiated generation. The young of this generation are the more slender "filariform embryos" (Fig. 111). Infection can occur either through these embryos of the free-living generation, or by direct transformation of rhabditiform into filariform embryos and these into the parthenogenic parasitic adult. (7) Trichocephalus Dispar (T. Trichiurus).--This, {259} the "whip-worm," is 4 or 5 cm. long. Its anterior portion is slender and thread-like, while the posterior portion is thicker (Fig. 112). It is widely distributed geographically, and is one of the most common of intestinal parasites in this country. It lives in the large intestine, especially the cecum, with its slender extremity embedded in the mucous membrane. Whip-worms do not, as a rule, produce any symptoms, although gastro-intestinal disturbances, nervous symptoms, and anemia have been ascribed to them. They, as well as many other intestinal parasites, are probably an important factor in etiology of appendicitis, typhoid fever, and other intestinal infections. The damage which they do to the mucous membrane favors bacterial invasion. [Illustration: FIG. 112.--Trichocephalus dispar; _a_, Female; _b_, male (natural size) (Heller).] The number present is usually small. The worms themselves are rarely found in the feces. The ova, which are not often abundant, are easily recognized. They are brown, ovoid in shape, about 50 µ long, and have a button-like projection at each end (Fig. 101). III. ARTHROPODA Most of these are external parasites, and the reader is referred to the standard works upon diseases of the skin for descriptions. The itch-mite (_Acarus scabiei_) and the louse (_Pediculus capitis_, _corporis_, _vel pubis_) are the more common members of the group.