The classification of euglenids is still variable, as groups are being revised to conform with their molecular
phylogeny. Classifications have fallen in line with the traditional groups based on differences in nutrition and number of flagella; these provide a starting point for considering euglenid diversity. Different characteristics of the euglenids' pellicles can provide insight into their modes of movement and nutrition. As with other
Euglenozoa, the primitive mode of nutrition is
phagocytosis. Prey such as
bacteria and smaller flagellates is ingested through a
cytostome, supported by microtubules. These are often packed together to form two or more rods, which function in ingestion, and in
Entosiphon form an extendable siphon. Most
phagotrophic euglenids have two flagella, one leading and one trailing. The latter is used for gliding along the
substrate. In some, such as
Peranema, the leading flagellum is rigid and beats only at its tip.
Osmotrophic euglenoids Osmotrophic euglenids are euglenids which have undergone
osmotrophy. Due to a lack of characteristics that are useful for
taxonomical purposes, the origin of osmotrophic euglenids is unclear, though certain morphological characteristics reveal a small fraction of osmotrophic euglenids are derived from phototrophic and phagotrophic ancestors. A prolonged absence of light or exposure to harmful chemicals may cause
atrophy and absorption of the chloroplasts without otherwise harming the organism. A number of species exists where a chloroplast's absence was formerly marked with separate genera such as
Astasia (colourless
Euglena) and
Hyalophacus (colourless
Phacus). Due to the lack of a developed cytostome, these forms feed exclusively by osmotrophic absorption. == Reproduction ==