Order 4. Peritricha, with only a ring of spiral cilia, mostly sedentary.
The Infusoria observed in man belong to the order Heterotricha, with few exceptions.
Genus. Balantidium, Claparède et Lachmann.
Heterotrichous Infusoria of oval or bag-like form and almost circular on transverse section; the anterior extremity narrowed, the posterior end broad and rounded off, or also narrowed; the peristome starting at the anterior end is broadest there and becomes narrower as it gradually obliquely approaches towards the posterior extremity. There are coarse cilia along the entire left border and the anterior part of the right border. Longitudinal striation is distinct and regular. There are two contractile vacuoles on the right, and occasionally also two or more to the left. The anus (cytopyge) is terminal. The macronucleus is usually horse-shoe or kidney-shaped, sometimes oval; the micronucleus contiguous. Reproduction is by binary fission and conjugation, and encystment occurs. The cysts are spherical or oval. These ciliates are parasitic in the large intestine of human beings and pigs, in Amphibia, and in the body cavity of polychæte Annelida.
Fig. 113.—Balantidium coli. a, nucleus; b, vacuole; c, peristome; d, bolus of food. (After Leuckart.)
Fig. 114.—Balantidium coli, free and encysted; a, anus or cytopyge; n, macronucleus; b, bolus of food. (After Casagrandi and Barbagallo.)