A re-description of the ciliate genus and type species, Balantidium entozoon.

Members of the ciliate genus Balantidium possess a specialized "Villeneuve-Brachon's" field of somatic cilia to the right of the vestibule, or in a dextroral location. Specimens of the type species were collected in Italy and fixed for study by light microscopy, and scanning and transmission electron microscopy. The presence of the field in the type species and several other species of the genus indicates a need to re-describe the genus by including details of the ultrastructure of that field. Scanning electron microscopy shows that the field consists of one row of relatively short cilia of uniform length flanked on each side by 2-3 rows, or more, of very short cilia. Their kinetids have typical litostome structure in transmission electron micrographs. We speculate on a possible function for the Villeneuve-Brachon's field and also present morphometric data on the type species. The base sequence of the small subunit ribosomal RNA gene of Balantidium entozoon has been determined and found to differ by 5% from that of B. coli. Based on the location and ultrastructure, organelles found around the somatic kinetosomes and within inter-kinetal ridges of B. entozoon were identified as hydrogenosomes.