Stomach "mouth". The words "gastro-" and "gastric" (meaning related to the stomach) are both derived from the Greek word "gaster" (, meaning "belly"). Although the precise shape and size of the stomach varies widely among different vertebrates, the relative positions of the oesophageal and duodenal openings remain relatively constant. As a result, the organ always curves somewhat to the left before curving back to meet the pyloric sphincter. However, lampreys, hagfishes, chimaeras, lungfishes, and some teleost fish have no stomach at all, with the oesophagus opening directly into the anus. These animals all consume diets that either require little storage of
Embryological origins of the mouth and anus that the structures are equivalent homologous, and that the older ideas about protostome mouth formation were correct. An alternative way to develop two openings from the blastopore during gastrulation, called amphistomy, appears to exist in some animals, such as nematodes. In many animals these early development stages later evolved in ways that no longer reflect these original patterns. For instance, humans have already formed a gut tube at the time of formation of the mouth and anus. Then the mouth is formed first, during the fourth week of development, and the anus is created four weeks later, temporarily forming a