The photo above is of what I believe is a testate amoeba from the genus Arcella. Testate amoebas are like normal amoebas, but are enclosed in a shell called a test. Towards the bottom of the cell is a single psuedopod, by which the amoeba achieves motility and apprehends its prey. The psuedopod extends from an aperture about the middle of the ventral side of the test, which is visible in the photo.
There are two types of tests in testate amoebas: Autogenic and xenogenic. Autogenic tests are composed of materials that the amoeba itself produced, and xenogenic tests are composed of materials that the amoeba collected (which sometimes includes diatoms). The test of Arcella is autogenic.