As prokaryotes, cyanobacteria lack a cell nucleus and other cell organelles, allowing their microscopic distinction from most other microalgae. The morphology of cyanobacterial cells shows a number of characteristics that can be used for microscopic examination and identification: primarily, the shape and size of cells, subcellular structures and specialised cells. Most cyanobacterial taxa form multicellular aggregates, and the size and shape of which can be used for the identification of cyanobacteria in freshly collected field samples. Cyanobacteria can produce a large diversity of secondary metabolites. For cyanobacteria, taxonomy is further complicated by the fact that two basically different systems of nomenclature have become established, the International Code of Nomenclature for algae, fungi, and plants and the International Code of Nomenclature of Bacteria. Cyanobacteria occur as unicellular, colonial or multicellular filamentous forms. The position, number and distribution of the heterocytes and akinetes are important morphological characteristics of species and genera.