When I started using well slides for observing microbes I knew that it would take longer for the water to evaporate, dramatically increasing the time that I could observe a particular sample. I found, however, that the farmiliar paramecium had vanished and that some globular creatures had appeared. I soon realized that the paramecium were "balling up" and becoming spherical.

This didn't explain why the paramecium were vanishing even before the water evaporated. I noticed that after balling up they started acting as if they were agaitated by something. Their movements gradually became slower overall but they would make sudden jerky movements on occasion. They also rotated in all directions showing that they were indeed spherical. Eventually their outside movements stopped but their insides continued to move and sometimes accelerated. The cilia stopped and eventually disappeared, either falling off or perhaps were absorbed.
Then, in a dramatic moment,one of them literally exploded right in front of my eyes!
So why do they ball up and then explode? I'm not sure. It could be the heat generated by the substage lighting or the lack of oxygen.