Internalization of macromolecules from the medium in Suctoria.

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Abstract
Tokophrya infusionum, like all other Suctoria, lacks on oral cavity. Its feeding apparatus consists of tentacles, long narrow tubes through which the contents of the living prey are ingested. For normal growth, reproduction and longevity of clones, Tokophrya needs supplements deriving from the medium in addition to living prey. Since Tokophrya lacks a mouth, these supplements can reach the cytoplasm only through the complex structure of the cortex, which is composed of a 3-membraned pellicle and a dense epiplasm. External to the cortex, an extraneous coat covers the whole organism. Only the outer pellicular plasma membrane is continuous; the other 2 and the epiplasm are interrupted by the outer plasma membrane which invaginates at intervals forming pits. The invaginated plasma membrane dips down into the cytoplasm where it extends to form a saccule. Experiments with cationized ferritin and Thorotrast provide evidence that internalization of these marcomolecules takes place through the pits by pinocytosis. The membrane of the saccules of the pits forms invaginations which pinch off giving rise to small, flattened vesicles containing the tracers. The tracers are never found free in the cytoplasm but exclusively in the flat vesicles. These vesicles are the vehicles transporting macromolecules from the medium to the cytoplasm. The saccules of the pits are the natural loci of pinocytosis and together with the flattened vesicles perform an important function in Suctoria, supplying the organisms with macromolecules from the medium.