The surface structure of the ciliary zone in 13 species of ciliates found in the large intestine of the horse was observed by scanning electron microscopy. In Holophryoides ovalis many fine depressions considered to be a result of phagocytosis or pinocytosis in the naked cytostome were noticed. In Blepharocorys spp. a distinct section was present between the portion with cilia and that without cilia. It was not present, however, in some species of the family Buetschliidae, such as Bundleia postciliata and Didesmis spp. The species of Entodiniomorphida had a lip around the ciliary zone with cilia forming synciliary tufts. In Spirodinium equi and Tetratoxum unifasciculatum the ciliary zone revolved counter-clockwise in an en face view. Some differences in the surface structure of the ciliary zone between the entodiniomorphid and spirotrich ciliates are discussed.

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