Conjugation.

http://en.wikipedia.org/wiki/Ciliate

Overview

During conjugation, two cells form a bridge between their cytoplasms, the micronuclei undergo meiosis, the macronuclei disappear, and the haploid micronuclei are exchanged over the bridge. In some ciliates (such as Vorticella), conjugating cells become permanently fused, and one conjugant is absorbed by the other.In most ciliate groups, however, the cells separate after conjugation, and both form new macronuclei from their micronuclei.Conjugation and autogamy are always followed by fission.