Feeding behavior in B. calyciflorus was observed in both constrained and unconfined individuals. There are at least three mechanisms by which the ingestion of suspended particles may be regulated. First, the cirri of the pseudotrochus may be extended, allowing particles to enter the funnel-shaped buccal field, or they may be bent over medially, forming a screen over the buccal field and preventing the entrance of even very small particles, such as yeast. Second, particles which do enter the buccal field may be rejected before being taken into the oral canal, presumably by a change in the ciliary beat. Third, particles accepted into the oral canal may be pushed back into the buccal field by the jaws and then rejected. The first two control mechanisms were observed to operate in unconfined rotifers attached to a surface or actively swimming through the medium, while the third mechanism was noted only at high magnification in constrained rotifers. B. calyciflorus employs different modes of feeding when in suspensions of the yeast Rhodotorula and in those of Euglena; pseudotrochal screening was seen only rarely in the former but communly in the latter.