FIG. 100.—Segments of—(1) Tænia saginata; (2) Bothriocephalus latus; (3) Tænia solium, showing arrangement of uterus.
FIG. 101.—Comparative size of eggs of intestinal parasites: a, Tænia solium; b, Tænia saginata; c, Ascaris lumbricoides; d, Trichocephalus dispar; e, Oxyuris vermicularis (after Strümpell).
FIG. 102.—Tænia echinococcus; enlarged (Mosler and Peiper).

(4) Tænia Echinococcus.—The mature form of this tape-worm inhabits the intestine of the dog and wolf. The larvæ develop in cattle and sheep ordinarily, but are sometimes found in man, where they give rise to echinococcus or "hydatid" disease. The condition is unusual in America, but is common in Iceland and Australia.

The adult parasite is 2.5 to 5 mm. long, and consists of only four segments (Fig. 102). It contains many ova. When the ova reach the digestive tract of man, the embryos are set free and find their way to the liver, lung, or other organ, where they develop into cysts, thus losing their identity. Other cysts, called "daughter cysts," are formed within these. The cyst-wall is made up of two layers, from the inner of which develop larvæ which are identical with the head, or scolex, of the mature parasite. These are ovoid structures about 0.3 mm. long. Each has four lateral suckers and a rostellum surmounted by a double circular row of horny hooklets. The rostellum with its hooklets is frequently invaginated into the body.

Diagnosis of echinococcus disease depends upon detection of scolices, free hooklets, or particles of cyst-wall, which is characteristically laminated and usually has curled edges. These can be found in fluid withdrawn from the cysts, or, less frequently, in the sputum or the urine when the disease involves the lung or kidney (Figs. [55] and 103).

FIG. 103.—Contents of echinococcus cyst, showing hooklets, scolices, and cholesterin crystals (Wood).

The cyst-fluid is clear, between 1.002 and 1.010 in specific gravity, and contains a notable amount of sodium chlorid, but no albumin.

2. Nematoda.—(1) Ascaris Lumbricoides.—The female is 20 to 40 cm. long and about 6 mm. thick; the male, a little more than half as large. Their color is reddish or brown. They are the common "round-worms" so frequently found in children. Their habitat is the small intestine. Large numbers are sometimes present.

FIG. 104.—Ascaris lumbricoides (female)
(Mosler and Peiper).