A foraminifer from the North Sea (left) and a ciliate from a ditch (right).
Even unicellular organisms can possess complicated structures. Foraminifers build up snail-like calcium carbonate shells that can reach a size of several hundred micrometers. Ciliates have not only cilia for locomotion. Some species have modified oral cilia to capture food by generating currents towards the mouth. The inside of the cell contains macro- and micro-cores as well as different vacuoles.