Euglenids are a group of mostly freshwater flagellates. Their mitochondria are unusual in both structure and molecular genetics. Many euglenids have firm but flexible pellicle and are capable of metaboly, a typical euglenoid movement.

Euglenids are well known for their nutritional modes diversity. The ancestral and most widespread mode of nutrition among euglenids is heterotrophy (bacteriovory, eukaryovory and primary osmotrophy). However, one monophyletic group, the euglenophytes, acquired a green secondary plastid and use photosynthesis as the main energy source. This plastid is derived from prasinophyte alga and has three envelope membranes. Light perceiving eyespot of unclear evolutionary origin is present in these organisms. The euglenophytes are still able to survive in dark by switching temporarily to heterotrophy; this feature this feature enabled the origin of several secondarily osmotrophic species with non-photosynthetic colorless plastids. Rapaza viridis, recently discovered mixotrophic lineage requires both photosynthesis and eukaryotic prey for survival.