A class of prokaryotic organisms coextensive with the division Cyanophycota of the kingdom Monera. Because these organisms have chlorophyll a and carry out oxygen-evolving photosynthesis, they have traditionally been aligned with algae and, with regard for their characteristic color, called blue-green algae. Microbiologists have emphasized the prokaryotic structure of these organisms and aligned them with bacteria, as the Cyanobacteria. Other names applied to these organisms include Cyanophyta at the level of division and Myxophyceae or Schizophyceae at the level of class. Blue-greens range in form from unicells 1–2 micrometers in diameter to filaments 10 cm (4 in.) long.