the hard, variously colored, calcareous skeleton secreted by certain marine polyps. 2. such skeletons collectively, forming reefs, islands, etc. 3. the solitary or colonial polyp that secretes this calcareous skeleton.
There are three major known groups of Archaebacteria: methanogens, halophiles, and thermophiles. The methanogens are anaerobic bacteria that produce methane. They are found in sewage treatment plants, bogs, and the intestinal tracts of ruminants.
Both archaebacteria and eubacteria are single-celled microorganisms, which are usually called prokaryotes. The main difference between archaebacteria and eubacteria is that archaebacteria are usually found in extreme environmental conditions whereas eubacteria are found everywhere on earth.