The Kingdom Monera consists of all prokaryotes, that is, unicellular organisms that lack nuclear membranes. This taxonomic kingdom consists of two phylogenetically distinct groups: eubacteria and archaebacteria. Morphologically, archaebacteria and eubacteria differ in some key aspects. While most members of both groups have cell walls, their cell membranes are chemically different, as are their overall chemical makeups. Though there is no hard and fast distinction between the roles filled by the two types of bacteria, most archaebacteria live in extremely hostile environments, such as extremely saline waters or hot sulfur springs. Some eubacteria also live in these harsh environments, but others inhabit locations ranging from surface soils to the intestinal tracks of termites. Though extremely small (most bacteria are significantly smaller than eukaryotic cells), bacteria fill several important roles in the natural world. We are most familiar with bacteria as the cause of diseases from strep throat to bubonic plague. However, comparatively few bacteria cause diseases. Most are beneficial to other organisms. The structure and reproductive cycles of the Monerans are relatively simple compared to those of the eukaryotes. They lack distinct nuclei and complex organelles. Specialized structures, such as photosynthetic machinery, take the form of internal membranes. Moneran genetic material is also relatively simple. They have small prokaryotic chromosomes and plasmids rather than the complex chromosomes found in eukaryotes. Most Monerans reproduce by binary fission.