The fine structure of the cytopharyngeal basket is described in detail. A circular palisade of cytopharyngeal rods is encircled at certain levels by a sheath and two annuli; crest-shaped structures project outwards from the basket. The rods, crests and sheath are largely composed of cytoplasmic microtubules of approximate diameter 240 Å. One of the annuli is mostly composed of densely staining inter-tubular material; the other consists mainly of fine fibres ranging in diameter from 40 to 90 Å.

Ingestion of algal filaments by Nassula and the sequence of movements of the rods which occurs during feeding are described. Evidence is presented which indicates that the crests and fibrous annulus both contract during feeding and act antagonistically to displace the apparently rigid rods. The initial displacement of the rods is spatially related to the orientation of the algal filament which is to be ingested. It is suggested that certain cilia respond to contact with the filament by transmitting some form of excitation to some of the crests along small bundles of microtubules which interconnect them. The suction force drawing the filament into the organism apparently acts only in the lumen of the basket. The basket seems to function as a fairly rigid structure which, acting in conjunction with the suction force, guides the filament into the organism and manipulates it to some extent. The basket also apparently grips the partly ingested filament to prevent it from slipping out of the organism during pauses in the action of the suction force.

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