Stomatogenesis of the Ciliate Genus Strombidinopsis with an Improved Description of S. spiniferum and S. acuminatum

Protargol‐stained cultured samples of Strombidinopsis spiniferum and S. acuminatum revealed a kind of enantiotropic mode of cell division. Stomatogenesis may be divided into eight stages. Stage 1 was the appearance of an anterior anarchic field of a few kinetosomes, probably originating apokinetally near a somatic kinety on the dorsal side of the cell. In Stage 2, the anarchic field becomes denser and larger by kinetosomal replication. The kinetosomes become aligned as oral polykinetids (OPk's) in Stage 3. In Stage 4, the paroral (PO) emerges when most of the anarchic field has transformed into OPk's. In Stage 5. all OPk's are apparently formed, and the anlage resembles a barrel, lacking a few staves. In Stage 6, the barrel rotates approximately 90° transversely to the cell's axis, concomitantly the outer end of the barrel splays open. In Stage 7, the somatic kineties start to divide, the anlage has opened to the cell surface, and the part with the internal OPk's spirals into the oral cavity while the OPk's form a closed circle. At this stage, one or two large macronuclei were seen preparing to divide. In Stage 8, the adult or interphase stage, the cytopharynx is formed. Based on our results, the original redescriptions of S. spinifierum and S. acuminatum must be modified in three respects: the cells have a PO; the OPk's are composed of three rows of kinetosomes (uncertain for S. acuminatum ), not two as originally reported; and the number of internal OPk's is not always 3, but ranges from 3‐4 in S. spiniferum and 3‐5 in S. acuminatum.