Pyloric caeca are finger-like projections located in the stomach of many fish species. The pyloric caeca secrete digestive enzymes and increase the surface area in the stomach for nutrient absorption. The Florida Fish and Wildlife Conservation Commission places the location of pyloric caeca near the junction of the stomach and intestines.
According to the National Academy of Science, Aristotle first theorized about the function of pyloric caeca in fish in 345 B.C. He hypothesized three purposes: storage, purification and digestion.
The National Academy of Science explains that modern research in trout, cod, largemouth bass and striped bass support the digestion hypothesis. The pyloric ceaca in these four species are major sites of sugar, amino acid and dipeptide absorption, which are all parts of digestion. The brush border membranes of the pyloric caeca also contain hydrolytic enzymes.
As defined by Biology Online, hydrolytic enzymes are proteins that use water to perform their digestive functions. In the observed fish species, the pyloric caeca fill and empty of food as a means to increase the amount of surface area available for the enzymes to do their work, thereby increasing nutrient absorption.
Further evidence to support Aristotle's idea is detailed in an article in Science Direct. A trypsin-like enzyme has been discovered in the pyloric caeca and intestines of some fish species. Trypsin is an enzyme produced in the pancreas, which acts on proteins by cleaving peptide bonds, aiding digestion.