In the cytoplasm of the ciliate Ichthyophthirius multifiliis, a parasite of the skin and gills of freshwater fishes, we found rod-shaped bacteria-like particles. These xenosomes look like Gram-negative bacteria and possess lighter areas with fibrillar material, which probably corresponds to bacterial DNA. Sometimes it was possible to observe dividing xenosomes inside the ciliate cytoplasm. Special reference is made to the assemblage of glycogen granules that surround the endocytobionts in the trophozoites, since this is the first report of an association between endocytobionts and glycogen. However, in theronts this association is not evident.
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