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.
Archaebacteria are a type of prokaryote, that is, a unicellular organism without a cell nucleus. They make up the kingdom Archae, one of the main kingdoms of life. These organisms are difficult to classify because they have similarities to both normal bacteria and the larger eukaryotes.