The stomach is a muscular, hollow, dilated part of the gastrointestinal tract that functions as an important organ in the digestive system. The stomach is present in many animals including vertebrates, echinoderms, insects (mid-gut), and molluscs. In humans and many other vertebrates it is involved in the second phase of digestion, following mastication (chewing). In most vertebrates, the stomach is located between the esophagus and the small intestine. It secretes digestive enzymes and gastric acid to aid in food digestion. The pyloric sphincter controls the passage of partially digested food (chyme) from the stomach into the duodenum where peristalsis takes over to move this through the rest of the intestines.