Schematics of the experimental apparatus. Test fluid was placed between the bottom of the inner dish and the top of the outer dish

directions slightly in the near field, and finally move away from each other. All the cells interactions were classified as hydrodynamic interactions, avoiding reactions or escape reactions. The total number of interacting cells recorded in the study was 602 (the total number of cases was 301). It was found that 84.7% of cells interact hydrodynamically. The ratio of es-cape reaction was slightly higher than that of avoiding reaction. We could conclude that the cell-cell interaction was mainly hydrodynamic and that the biological reaction accounted for a minority of incidents

Sample sequences showing the hydrodynamical interactions when two swimming P. caudatum experience a near-contact. The time interval between each sequence is 1/3 sec. Long arrows are added to schematically show cell motion. Scale bars, 500 jim