Planeticovorticella finleyi n.g., n.sp. (Peritrichia, Vorticellidae), a planktonic ciliate with a polymorphic life cycle

. Free‐swimming trophonts of a sessiline peritrich ciliate were discovered in plankton samples from the Rhode River, Maryland, and main‐stem Chesapeake Bay. Cultures revealed that the species comprises both free trophonts that swim with their peristomial cilia and sessile trophonts that attach to substrates with a contractile, helically‐twisted talk. Trophonts with a short, rigid stalk or no definite stalk also were seen in culture. Binary fission of free‐swimming trophonts usually produced a pair of trophonts attached scopula to scopula by a short, rigid stalk. These persisted for some time as distinctive, spinning doublets before their stalks broke and they separated. Binary fission of free‐swimming trophonts also yielded trophont‐telotroch pairs that stayed together for only a short time. Telotrochs from these pairs were presumably the source of attached trophonts. Conjugation occurred in both free and attached trophonts. Formation of microconjugants involved at least 2 successive divisions of a trophont. Possession of a helically‐twisted, contractile stalk placed the peritrich in the family Vorticellidae, but its unique combination of life‐cycle stages marks it as a new genus and species, Planeticovorticella, finleyi The morphology and life cycle of P. finleyi raise questions about the present classification of sessiline peritrichs and suggest that it may be at least partly artificial. Stalkless planktonic peritrichs that swim with their oral cilia as do trophonts of P. finleyi may have evolved from sessile ancestors by an alteration in the life cycle that created unstable clusters of trophonts on a single parental stalk. Free‐swimming trophonts would originate from breakup of these clusters.