SUMMARY

The flagella-like, but immobile, pseudocilia of the unicellular green alga, Tetraspora lubrica, have been characterized ultrastructurally and compared with the true flagella of the motile green alga, Chlamydomonas. The basal body and transitional regions of the 2 organelle systems are basically similar. Most of the marked differences occur in the pseudocilium proper, and a few important dissimilarities are found in the cytoplasmic portions of the apparatus. The major differences are: (1)4 instead of 2 proximal striated fibres in the pseudociliary apparatus; (2) a proximal-to-distal progressive decrease in subfibre number from 9 doublet fibres to a solitary fibre in the pseudocilium proper; (3) loss of interconnexions among the subfibres in the pseudocilium proper and a concomitant disorientation of those subfibres; (4) absence of major arms (dynein) on the A subfibre of peripheral doublets; (5) absence of the central pair of fibres. These and other differences are discussed with regard to their possible roles in loss of motility. Additional ultrastructural details- interconnexions between cytoplasmic microtubules and basal body subfibres of pseudocilia and flagella- are described and discussed in terms of their possible roles in the regulation of flagellar movement in motile organisms