Remember our testate amoeba friends, the arcellinids? Here is a pair of Arcellas (Arcellae?) in the midst of division. Organic tests(=”shells”) rust over time, as in they turn yellow and then brown with oxidation. Based on that, you can tell that the newer test is on the bottom, as they start out clear. These cells have almost finished dividing, with a lingering cytoplasmic bridge barely connecting the two, between their mouthes (ok, ‘oral apertures’). Mouth-to-mouth division. The three tiny round things at the top end of the cytoplasmic bridge look like the last organelles that will be transferred over to the younger cell — perhaps a few mitochondria, based on their size. Don’t worry, there’s already plenty that have been transferred to the younger cell already!
To split, arcellinids need to orient themselves on their sides, perpendicular to the surface. They do this by producing gas bubbles that increase bouyancy at one end and cause them to re-orient. The bubble at the bottom of the younger cell could be a remnant of that, or could be just a contractile vacuole — can’t tell from this image. Next to it is a nucleus with a very obvious nucleolus. The top cell has one too. After separating and returning to their default upright position, the two amoebae will probably be quite hungry, and go on with their ploy to devour pretty much anything they can get on top of. There are documentations of arcellinids feasting on nematodes and rotifers, but more on that sometime later. This specimen is from a pond sample, although they thrive in soil as well.