Euglenoids form a diverse group of eukaryotic organisms that may have arisen a billion years or more before the origins of plants and animals.They are largely unicellular flagellates with only one genus, and they form colonies. There are nearly 2,900 nominal species for the major photosynthetic taxa, but, of these, only about 800 are accepted as taxonomically valid. Modern phylogenetics has caused numerous changes in euglenoid taxonomy, which keeps the number of accepted species in constant transition.