This is a “test,” a structure built by an amoeba as protection from its environment. You can see a couple of pseudopods (“arms”) of the amoeba at the top and top-right of the test, stretching out of an opening in the test for locomotion and feeding.

The test is constructed of mineral grains, portions of diatom silica shells and other material in the environment, and held together with an organic glue. It has 8 hollow, conical spines (some are hidden behind others in this photo). The amoeba species that builds this specific test is Netzelia corona. The water sample came from a freshwater pond in Satun, southern Thailand. For reference, the 80 microns shown on the scale is the width of an average human hair.

This image is a focus stack of frames from a 1-1/2 second video, traveling from one end of focus to the other.