This page shows two different types of one-celled organisms belonging to the group called ciliates. Vorticella, from fresh water, and a marin tintinnid Parafavella. Another group called radiolar (marine) has a little image down on the page.
The one-celled animals plays a relatively minimal role in the zooplankton. The size is ordinary less than 100 my or 1/10 mm. The one-celled organisms have yet different functions in the cell. The upper part of Vorticella and the tintinnid called Parafavella have cilia rim around an opening. The cilia are beating and produce a current of water with nutrients to the cell.
Video. Vorticella is often found in colonies, probably caused by divisions of one individual.
Tintinnids - parafavella from the sea
Tintinnids have relatively big cells characterized by a house of hard organic material and where the single cell is inside. There are several species of tintinnids. The size can be 200 my. The cell has a cilia rim in the one end and a stalk in the other end, which is attached to the house. The moving cilia make a water stream to the opening of the cell and nutrition particles are captured by the cilia.
Video 1 Parafavella. The dinoflagellate Ceratium have almost the same size.
Video 2 details of the house and the stretching of the cell.
Radiolaria from sea
A radiolaria is shown. The cell is inside a skeleton of siliceous with radiating spicles. Their skeletons often form siliceous ooze on the seabed.