Digestive system - a system of organs that provide mechanical grinding and breakdown of complex organic substances to the form most suitable for absorption and participation in metabolism. It is characterized by a strongly pronounced partitioning into sections (except for roundworms) and the presence of isolated digestive glands. The great diversity in the digestive system of bony fishes is primarily associated with the nature and mode of feeding.

On the internal structure of fish are similar to other vertebrates. The body is bilaterally (bilaterally) symmetrical, except for the digestive tract. The latter consists of the mouth, jaws, usually covered with teeth, tongue, pharynx, pharynx, esophagus, stomach, intestines, pyloric appendages, liver, pancreas, spleen, rectum, or large intestine and posterior, or anal, opening.

The intestines of sharks and some other primitive fish contain a spiral valve, a unique organ that increases the "working" surface of the digestive tract without increasing its length. In predatory fish, the intestines are usually short, forming one or two loops, while in herbivorous species they are long, winding, with many loops.