Thin sections of rumen ciliated protozoa of the subclass Holotrichia were observed with a transmission electron microscope. These protozoa had a double layered boundary separating the ectoplasm from the endoplasm. Starch granules were abundant throughout the endoplasm and are probably storage starch. Also in the endoplasm of the holotrichs were the macro- and micronuclei. They were adjacent to one another, apparently surrounded by a continuous membrane. Many unidentified dense bodies appeared in the endoplasm adjacent to the inner layer of the ectoplasmic-endoplasmic boundary. These inclusions could be precursor material to the boundary. The cuticle contained granular inclusions which might be secreted to facilitate ciliary movement. The holotrichs ingested chloroplasts as these were in vacuoles throughout the cytoplasm. The anal pore appeared to be open to the exterior and lined by a unit membrane.
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