Brachiopods are marine animals with one upper and one lower shell.
The right and left halves of each shell are mirror images, but the upper and lower shells are not exactly alike. The shells commonly are composed of calcium carbonate or calcium phosphate and they range in size from less than a fourth of an inch to several inches. Brachiopods are uncommon in most oceans today, but at times in the past they were the most abundant shellfish and sometimes formed large shell banks, much as oysters do today. Like modern ones, most ancient brachiopods lived attached to the sea floor by a fleshy stalk that was an extension of the soft body. Some forms lost the stalk when they became adults and either attached themselves directly to the sea floor or lay loose in the mud or sand.