In this holotrichasin, hymenostomorid family, the buccal ciliature is composed of 3 membranelles which lie to the left in the buccal cavity and a fourth, paroral membrane extending along its right margin. One or more stomatogenous rows of cilia end at the posterior margin of the buccal pouch.
The body is piriform and uniformly ciliated with 17 to 42 rows of cilia. The piriform cytostome is near the anterior end. There is a single contractile vacuole.
Tetrahymena pyriformis (Ehrenberg, 1830) Lwoff, 1947 (syn., T. geleii) measures 40 to 60 by 15 to 30 u. It is extremely popular in protozoological research. According to Corliss (1954, 1957a), over 500 papers had been written on it and other members of the genus up to 1954, and another 186 papers were published in 1954 thru 1956. Altho T. pyriformis is normally free-living, it may on rare occasions be a facultative parasite. Knight and McDougle (1944) found it in the digestive tract, infraorbital sinuses and serous material under the eyelids of chickens in Missouri. It was found only in birds with a vitamin A deficiency.
Thompson (1958) infected chicken embryos with T. pyriformis, T. corlissi and T. vorax. He also infected guppies (Lebistes reticulatus) and tadpoles (Rana palustris) thru artificially produced wounds with T. corlissi but not with the other species. Various adult and larval insects proved excellent hosts, the protozoa teeming in the hemolymph of some of them.