The stomach is a muscular, hollow, dilated part of the digestive system which functions as an important organ of the digestive tract in many animals, including vertebrates, echinoderms, insects (mid-gut), and molluscs.he stomach lies between the esophagus and the duodenum (the first part of the small intestine). It is in the left upper part of the abdominal cavity. The top of the stomach lies against the diaphragm.