When early cells began to produce oxygen, its accumulation forced the appearance of cells able to tolerate it and utilize carbon compounds for energy. The protistans are like those early eukaryotic cells. Today we include both single-celled and multicelled forms in this group.
Protistans are a collection of the simplest eukaryotic organisms.
Both single-celled and multicelled forms are included. In single-celled species, the spores are motile. More complex species develop mycelia, masses of absorptive filaments whose cells do not have cross walls. They produce extensive mycelia, some of which become modified to form gamete-producing structures. The diploid zygote develops into a resting spore that will germinate into a mycelium-producing motile, asexual spores.