Brachiopods are one of the oldest groups of animals and are divided into two classes: Unarticulated and Abdominal. It is estimated that there are about 3,000 genera (including about 150 genera in the Aphids) and 30,000 species, most of which lived in the Paleozoic era. There are only 300 living species, belonging to 74 genera. The characteristic of brachiopods is that they have two shells, of equal or different sizes, covering the dorsal and ventral sides.