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 microscopic organisms with diameters ranging from 0.0002â0.0004 in (0.5â1.0 micrometer). The volume of their cells is only around one-thousandth that of a typical eukaryotic cell. They come in a variety of shapes, which can be characterized into three common forms.