Frontonia are a large, gluttonous species of ciliates, which makes them a wonderful, colourful, subject of microphotography. Despite the modest appearance of their mouth, they can swallow some impressively big prey -- the suture beneath the mouth can open to widen the engulfment. Essentially, the critter unstitches its belly to fit more in. Would be nice for exceptionally big dinners, eh?
You can see a clear, slightly bubbly, macronucleus -- a thick wad of its own DNA. Beneath the nucleus is a freshly engulfed diatom -- you can tell it's fresh by its intact state, particularly of its plastids. Above the nucleus is an example of what will soon happen to it -- dissolve in acid and enzymes of a digestive vacuole. This vacuole will soon be recycled back to the surface to dispose of the victim's remains. The membrane becomes part of the cell surface, and then "spent" as phagocytic vacuoles (newly minted food vacuoles) form at the mouth. A protist's circulatory system can be a little more exciting than ours, in a way.
Here's a labelled version for clarification:
I haven't quite figured out the identity of the refractile crystaline blob near the centre, above the "mouth" (inside a vacuole). If anyone knows -- I'd love to hear it!