Syn.: _Paramæcium coli_, Malmsten, 1857. [Illustration: FIG. 113.--_Balantidium coli._ _a_, nucleus; _b_, vacuole; _c_, peristome; _d_, bolus of food. (After Leuckart.)] [Illustration: FIG. 114.--_Balantidium coli_, free and encysted; _a_, anus or cytopyge; _n_, macronucleus; _b_, bolus of food. (After Casagrandi and Barbagallo.)] The body is oval, 60 µ to 100 µ in length (up to 200 µ according to Janowski), and 50 µ to 70 µ in breadth. The peristome is funnel-shaped or contracted, the anterior end being then broadened or pointed according to the degree of contraction (figs. 113, 114). The ecto- and endo-plasm are distinct, the latter is granular, containing drops of fat and mucus, granules of starch, bacteria, and occasionally also red and white blood corpuscles. There are usually two contractile vacuoles, seldom more. The anus (cytopyge) opens at the posterior extremity. The macronucleus is bean- or kidney-shaped, rarely oval; the micronucleus is spherical. _Balantidium coli_ lives in the large intestine of man, in the rectum of the domestic pig, and has been found in monkeys. It propagates by transverse division, but conjugation and encystment are known to take place.[247] Transmission to other hosts is effected by the cysts of the parasite (fig. 114). [247] According to Gourvitch (“Bal. coli. Darmk. d. Menschen,” _Russ. Arch. f. Path., klin. med. u. Bact._, Petrograd, 1896), the conjugated Balantidia are supposed to fuse with each other and form oval cysts two or three times the size of the free organisms, and to divide into numerous globules within the cystic membrane; the process, however, has hitherto not been confirmed. The supposed Balantidium cysts appeared in two patients who were simultaneously suffering from _Dibothriocephalus latus_, after the administration of anthelminthics. It therefore seems, according to the description, that in reality these forms were actually abnormally large, possibly swollen, young eggs of the tape-worm mentioned. _Balantidium coli_, first seen by Leeuwenhoek, was described by Malmsten in 1857 in a man aged 35 years, who had two years previously suffered from cholera, and since then had been subject to diarrhœa. The examination showed an ulcer in the rectum above the mid sphincter ani, in the sanguineous purulent secretion of which numerous Balantidia were swimming about. Although the ulcer was made to heal, the diarrhœa did not cease and the stools contained numerous Balantidia, the number of which could only be decreased by extensive enemas of hydrochloric acid. The second case related to a woman who was suffering from severe colitis, and who died ten days after admission. The malodorous, watery evacuations contained innumerable Balantidia, in addition to pus, and at the autopsy the anterior portion of the large intestine was found to be infested with them.