[Illustration: FIG. 94.--Balantidium (Paramoecium) coli (Eichhorst).] 4. Hemosporidia.--This is a large group of parasites with two life-cycles: one in the blood-corpuscles or plasma of a vertebrate host--man, mammals, birds, reptiles; the other in the body of some insect. The malarial parasite, already described; the organism (_Pirosoma bigeminum_) producing Texas fever in cattle; and the questionable parasite (_Piroplasma hominis_), which has been described as the cause of "tick fever" of Montana, belong to the group. 5. Trypanosomes have been mentioned (p. 195). II. VERMES 1. Cestoda.--Tape-worms are very common parasites of both man and the animals. The most important are _Tænia saginata_, _Tænia solium_, _Bothriocephalus latus_, and _Tænia echinococcus_. They all pass a larval stage in the body of an intermediate host. In the adult stage they consist of a linear series of flat, rectangular segments (proglottides), at one end of which is a smaller segment, the scolex or head, especially adapted for attachment to the host. The series represents a colony, of which the {248} scolex is ancestor. The proglottides are sexually complete individuals, derived from the scolex by budding. With exception of the immature segments near the scolex, each contains a uterus filled with ova. The three tape-worms first mentioned are distinguished from one another mainly by the structure of the scolex and of the uterus (Fig. 100). The scolex should be studied with a low-power objective. The uterus is best seen by pressing the segment out between two plates of glass. [Illustration: FIG. 95.--Tænia saginata (Eichhorst).] (1) Tænia Saginata or Mediocanellata (Fig. 95).--This, the beef tape-worm, is the common tape-worm of the United States. Its length sometimes exceeds twenty-five feet. The middle segments measure about one-fourth by one-half inch. The scolex is about the size of a pin-head, and is surrounded by four sucking discs, but has no hooklets (Fig. 96). The uterus extends along the middle line of the segment and gives off about twenty branches upon each side (Fig. 100). The larval stage is passed in the muscles of various animals, especially cattle, where it lies encysted (cysticercus stage). The larva is ingested with the meat, its capsule is {249} dissolved by the digestive juices, and it attaches itself to the intestinal wall by means of its suckers. It then develops into the mature worm. The ova are present in the stools of infected persons, often in great numbers. They are spheric or ovoid, yellow in color, and have a thick, radially striated shell (Fig. 101). Their greatest diameter is 30 to 40 µ (about four or five times the diameter of a red blood-corpuscle). Vegetable cells, which are generally present in the feces, are often mistaken for them. [Illustration: FIG. 96.--Head of Tænia saginata (Mosler and Peiper).] [Illustration: FIG. 97.--Head of Tænia solium (Mosler and Peiper).]