Ultrastructural and other observations which suggest suctorian affinities for the taxonomically enigmatic ciliate Cyathodinium.

Abstract

SYNOPSIS. Two species of the taxonomically enigmatic genus Cyathodinium, C. piriforme and C. cunhai, were studied in some detail at both light and electron microscopic levels. Data obtained strongly suggest suctorian affinities for the genus, since a number of structures or features are strikingly reminiscent of similar (if not homologous) structures recently discovered in ciliates belonging to the order Suctorida.Endosprits (suctorial tentacles?) of Cyathodinium show an arrangement of microtubules not unlike that known for several suctorians, especially Acineta and Tokophrya. Haptocysts or missile‐like bodies, ca. 600 mμ long, have been observed within endosprits and free in the cytoplasm; again this is reminiscent of the complex organelles recently described from several suctorian groups. Mouthlessness, coupled with the presence of a ventral depression (functioning in gathering prey at distal ends of endosprits?) and the presence of food vacuoles in the cytoplasm, further support a suctorian mode of feeding. Finally, stages in the curious life cycle of Cyathodinium suggest neoteny and a basic similarity to endogenous budding processes in certain suctorians.