(2) Tænia solium, the pork tape-worm is very rare in this country. It is usually much shorter than _Tænia saginata_. The scolex is surrounded by four sucking discs, and has a projection, or rostellum, with a double row of horny hooklets (Fig. 97). The uterus has only seven to ten branches (Fig. 100). The ova closely resemble those of _Tænia saginata_, but are a little smaller (Fig. 101). (3) Bothriocephalus latus, the fish tape-worm, is the largest parasite of man, sometimes reaching fifty feet in {250} length, although generally not more than half so long. It is common in some countries of Europe and in Japan, but is very rare in this country. The head is not unlike the bowl of a spoon in shape. It is unprovided with either suckers or hooklets, but has two longitudinal grooves which serve the same purpose (Fig. 98). The uterus, which is situated in the center of the segment, is roset-shaped (Fig. 100). [Illustration: FIG. 98.--Head of Bothriocephalus latus: _a_, _a_, Bothridies; _b_, neck (Blanchard).] The larval stage is found in fish, especially the pike. [Illustration: FIG. 99.--Egg of Bothriocephalus latus, showing lid and yolk granules (photograph by F. C. Wood).] The ova are characteristic. They measure about 45 by 70 µ, are brown in color, and are filled with small spherules. The shell is thin, and has a small hinged lid at one end (Fig. 99). {251} Bothriocephalus latus is interesting clinically because it often causes a very severe grade of anemia. [Illustration: FIG. 100.--Segments of--(1) Tænia saginata; (2) Bothriocephalus latus; (3) Tænia solium, showing arrangement of uterus.] [Illustration: 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).] (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 {252} 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. [Illustration: FIG. 102.--Tænia echinococcus; enlarged (Mosler and Peiper).] 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.