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SYNOPSIS. In the culture forms of the elasmobranch trypanosome Trypanosoma raiae is found a prominent cytopharyngeal complex. This consists of a group of 5 or 6 microtubules associated with a deep invagination of the cell membrane which arises from a cytostome near the opening of the flagellar pocket. This structure is a constant feature of the various epimastigote and trypomastigote forms that this flagellate has in culture. Replication of the cytopharyngeal apparatus is completed before cytokinesis. Experiments using ferritin as an electron dense tracer show that endocytosis occurs from the blind ending of the cytopharynx both in the exponential and stationary phases of growth in vitro. Ferritin is transported from the cytopharynx by endocytotic vesicles to large, membrane‐bound vacuoles in the posterior region of the cell. Ultrastructural location of non‐specific acid phosphatase within these digestive vacuoles and also within the Golgi apparatus is reported. Coated vesicles found in association with the flagellar pocket are another route of uptake of ferritin by T. raiae.

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The amoeboid locomotion of Acunthumoebu custellunii has been studied by observation of individual cells moving on a planar glass substratum. Cell-substratum interactions involved in traction have been observed by reflexion interference microscopy. A variable part of the ventral surface of A. custellunii formed a protean platform, the 'associated contact', from which filopodia were subtended ; these established stable, focal adhesions (approximately 0.4 pm diameter) on the substratum beneath. Surprisingly, acanthopodia, a prominent feature of this protozoon, did not play an obvious role in traction. The dimensions of the cell-substratum gap in the associated contact could be modulated by the concentration of ambient electrolyte. Dilution of electrolyte from 50 mM-KCl to 2 mM resulted in (i) an increase in the cell-substratum gap, (ii) a marked decrease in cell motility, (iii) reduced cell adhesion to glass.

Acanthamoeba castellanii (CCAP 1534/3) was found to bind avidly the common soil bacterium Pseudomonasfluorescens. This adhesion was mediated not by pili nor by the general bacterial surface but by the polar flagella. Because of the nature of the flagellar rotary motor, the cell bodies of the attached bacteria could be seen rotating clearly. While initially bacterial binding occurred uniformly over the cell membrane of Acanthamoeba, the bacteria were soon swept posteriorly to form a cap and either endocytosed or sloughed off, still agglutinated by their flagella. Such capped amoebae would not bind Pseudomonas if challenged immediately, indicating a depletion of flagella-binding sites. The bacteria could not bind to amoebae pretreated with concanavalin A (Con A) even after the lectin had been capped to the uroid. However, capping of flagella-binding sites did not co-cap all the Con A-binding sites on the surface of the amoeba. The flagella-binding sites were not affected by pre-treatment with Pronase (1 mg ml -I ) or anti-Acanthamoeba surface antibody. Pruteus mirabilis also bound avidly by its flagella to Acanthamoeba and, furthermore, competition experiments suggested that Pruteus and Pseudomonas adhere to a common surface site on the amoeba. The presence of sites on the cell membrane of A . castellanii that are specific for flagellin would enhance strongly the adsorption of motile bacteria prior to endocytosis. This would represent an excellent feeding strategy for a soil-dwelling phagotroph.Abbreoiutions : Con A, concanavalin A ; FITC, fluorescein isothiocyanate.