The prokaryotic division containing the blue-green *algae
. It contains unicellular, colonial, and filamentous forms, many of which are surrounded by a mucilaginous sheath, hence the older name Myxophyceae. Sexual stages and flagellate forms are unknown. The predominant pigment is phycocyanin, which confers a wide range of different colours depending on species and environment. Blue-green algae are abundant in fresh water and the soil; they also occur in the marine littoral zone, in hot springs, as symbionts in lichens and root nodules, and as parasites in certain plants. The nuclear and plastid material of-blue-greens is not bound by membranes, chromosomes are lacking, and the pigments are dispersed in a primitive chro-matophore. Some can also fix nitrogen. These characteristics have led many to classify them with the bacteria under various names, such as Cyanobacteria and Schizophyceae. However they differ from bacteria in their lack of motility and lack of fermentative activity. The presence or absence of *heterocysts
and hormogonia and the tendency of cells to aggregate into distinct filaments are the main features by which orders are recognized. The *Chroococcales
and the *Hormogonales
are the most important orders.