Movement of the cirri during the creeping of Euplotes crassus (Ciliata, Hypotrichida)

The beating cycle of several cirri (frontal cirri 1/0, 3/I, 3/II, 3/III; tr ansverse cirri; one caudal cirrus) of the ciliate Euplotes crassus was stud ied and described thoroughly in specimens that were actually creeping along the substrate. The beating cycle of the frontal cirri was measured both sp atially and temporally, and it was found that (i) the single beating cycle was formed by an active propulsion phase (about 70% of the single step), fo llowed by a recovery phase that so far has never been described and is wher e the cirri are transferred forwards passively (about 30% of the step); (ii ) whenever the euplotes stops, it assumes its "zero position," repositionin g all of its frontal cirri to their respective "standard positions"; and (i ii) at the beginning of a new creeping phase the frontal cirri were reactiv ated in a well-defined order. The transverse cirri were kept still during f orward creeping, while their angular position was changed with respect to t he substrate during the stops and backward movements of the ciliate. The fi rst left caudal cirrus beats constantly and its operating cycle appeared to be independent of the creeping or immobile state of the organism. The find ings are discussed from the functional point of view and in the context of available literature on the internal beating potentialities of the differen t cirri.