The three-dimensional particle paths due to a helical beat pattern of the flagellum of a sessile choanoflagellate, Salpingoeca Amphoridium (SA), are modelled and compared to the experimental observations of Pettitt (2001). The organism’s main components are a flagellum and a cell body which are situated above a substrate such that the interaction between these entities is crucial in determining the fluid flow around the choanoflagellate. This flow of fluid can be characterized as Stokes flow and a flow field analogous to one created by the flagellum is generated by a distribution of stokeslets and dipoles along a helical curve.

The model describing the flow considers interactions between a slender flagellum, an infinite flat plane (modelling the substrate) and a sphere (modelling the cell body). The use of image systems appropriate to Green’s functions for a sphere and plane boundary are described following the method of Higdon (1979a). The computations predict particle paths representing passive tracers from experiments and their motion illustrates overall flow patterns. Figures are presented comparing recorded experimental data with numerically generated results for a number of particle paths. The principal results show good qualitative agreement with the main characteristics of flows observed in the experimental study of Pettitt (2001).