Algae are classified on the basis of conservative characteristics such as the type of cell wall, flagella, photosynthetic pigments, and the form in which food is stored. Many dinoflagellates are colorless and live as heterotrophs and parasites. Some produce nerve poisons toxic to vertebrates. Many euglenoids contain two flagella and contain a hard pellicle made of protein just under the plasma membrane. Many contain a red eye spot, which is thought to be used as a photoreceptor.
Euglenoids reproduce asexually by dividing lengthwise into two.
They do not reproduce sexually. They are autotrophic and heterotrophic. Euglena Golden algae occur as single cells or as colonies of great diversity and complexity. Most are freshwater, a few marine. Some lake dwelling golden algae are both heterotrophic and autotrophic.