An electron microscope study was made of Deltotrichonympha and Koruga, two closely-related hypermastigote flagellates that live in the hindgut of the Australian termite, Mastotermes darwiniensis These symbiotic protozoans have a typical flagellated rostrum and long body flagella. Their "giant centrioles" (centriolar apparatus) are large, fibrillar, and granular bodies which do not resemble typical centrioles in structure. The unique feature of interphase cells is the presence of more than half a million free kinetosomes in the anterior cytoplasm. Two classes of free kinetosomes, differing in length and spatial arrangement, were found. 500,000–750,000 short free kinetosomes are concentrated in a dense column which extends from the centriolar apparatus in the rostrum to the anterior side of the nucleus Most of the short free kinetosomes in the column are arranged end-to-end in chains of varying lengths. Within a kinetosomal chain, all of the individual kinetosomes face in the same direction with respect to their cartwheel ends In most flagellates, the short free kinetosomes are 0 07–0.13 µ long, and are remarkably similar in length within any cell Occasionally, cells with uniformly "longer" short free kinetosomes are found. 70,000–120,000 long free kinetosomes are scattered singly throughout the cytoplasm between the column of short free kinetosomes and the cell surface These long free kinetosomes are 0 4–0 7 µ long, similar in length to the kinetosomes of the body flagella, and are oriented parallel to the anterior-posterior axis of the cell. The significance of this remarkable accumulation of free kinetosomes is discussed.

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