The Opalinid Infusorians: Flagellates or Ciliates?

SUMMARY. The controversial question of the affinities of the enigmatic opalinids is discussed in the light of recent findings and with regard for modern ideas concerning characters of most significance in the phylogeny of various major protozoan groups. Metcalf's “protociliate” hypothesis still retains a high popularity although many of the factors supporting it have been adequately disproven during the past 20 years. The “ciliate‐” and “flagellate‐like” characteristics of the Opalinidae are listed and reviewed. Considered to be one of the most significant of mastigophoran features is the “symmetrigenic” mode of fission, a character generally disregarded in the past. Coupled with nuclear monomorphism and a syngamous reproductive cycle the type of division presents a very strong argument in favor of closer affinities with the zooflagellates than with the ciliates. It is stressed that the opalinids are highly differentiated forms, not likely primitive or ancestral for any major group of protozoa, which have had a long evolutionary history of their own. Grassé's recent erection of a new super‐order in the Zoomastigophora to contain the family Opalinidae is defended and commended.