Organisms in this domain are prokaryotic and live in extreme environments. Any of a group of single-celled prokaryoticorganisms (that is, organisms whose cells lack a defined nucleus) that have distinct molecular characteristics separating them from bacteria (the other, more prominent group of prokaryotes) as well as from eukaryotes.
Archaea evolved many cell sizes, but all are relatively small. The thermoplasmas are the smallest of the archaea. Most archaea fall into size classes (0.1 to 15 μ diameter and up to 200 μ long) matching bacteria. So they are about the size of a mitochondrion in a eukaryotic cell.Archaea have also evolved into many cell shapes similar to those of bacteria. There are bacilli, cocci, spirilli, and plate-like forms of archaea.Also like bacteria, the cells of archaea form various associations within a population of cells. Some species are unicellular, others are colonial, and yet others are filamentous.Just like bacterial cells, archaea cells come in three basic forms in terms of the cell boundary.
Bacteria definition, ubiquitous one-celled organisms, spherical, spiral, large group of microorganisms comprising one of the three domains of living organisms.