This bachelor thesis focuses on a detailed description of euglenids (Excavata: Euglenozoa), and selected important species from the class Euglenophyta. Euglenids are unicellular mostly free-living flagellates. Their most closely related protozoans are those from the group called Kinetoplastida. Members of Euglenophyta have specific and unique cell structure. A specific organelle, the stigma (also called eye spot), formed by a large number of pigment granules, helps them to perceive the direction from which light comes. An important part of the cell are also plastids, which cause different colouration of euglenids, and thus also colour of freshwater bodies in which they occur. Mobile stages of euglenids possess two flagella (sometimes only one flagellum is present - e.g. genus Euglena) which are anchored in small cavity called ampula. The surface of the flagellum is covered with fine hairs, mastigonemes. The thesis closely focuses on three euglenid species. Euglena viridis is characterized by its green colour and forms characteristic green clusters visible on the water surface of water bodies. Euglena gracilis is of great importance as a potential source in biotechnological use. It is one of the future sources of biofuels or healthy bioproducts. The last closely described model representative is...