EsophagusA short tube from the throat to the stomach. Food passes straight through it.
StomachDigestion begins here, in the fluids made by its walls. What a fish last ate can be found in it.
Pyloric cecaFinger-like pouches that open into the intestine just past the stomach. They add surface for digesting and absorbing food. A perch has three.
IntestineIt finishes breaking down the food and absorbs it. Fish that eat other fish have fairly short intestines; plant eaters need longer ones.
Anus (vent)The opening where undigested waste leaves the body. It lies just ahead of the anal fin.
LiverIt lies in the front of the body cavity. It makes bile, which helps digest fats, and it stores fats and carbohydrates.
GallbladderIt stores the bile made by the liver. The bile then passes into the intestine through the bile duct.
Swim bladderA gas-filled sac in the upper part of the body cavity. It matches the weight of the fish to the water, so the fish can hold its depth without swimming.
KidneyA long, narrow organ just under the backbone. It filters waste from the blood and controls water and salt in the body.
Urinary bladderIt stores the waste from the kidneys for a time. The waste then leaves through an opening just behind the anus.
GonadThe reproductive organ: an ovary in females, a testis in males. Most fish have a pair.
GillsFour gill arches on each side. Each has a double row of filaments rich in blood. Water from the mouth passes over them and oxygen is taken up.
HeartIt pumps the blood forward to the gills. It lies low in the throat, under them.
Outside the fish 10
MouthFood is caught with the jaws. Many small cone-shaped teeth hold it. It is not chewed.
EyeRounder than a mammal's eye. A fish focuses by moving its lens in and out, not reshaping it.
Operculum (gill cover)A flexible bony plate that covers and protects the gills. Water taken in at the mouth leaves from beneath it.
Dorsal finsThe fins on the back. In a perch the front one is held up by stiff spines and the rear one by soft, branched rays. They help keep the fish from rolling over.
Lateral lineA row of pores along the side that leads to sense organs. It detects currents, pressure and motion in the water.
ScalesThin plates that overlap like the shingles on a roof. They protect the body. A perch's scales have a toothed rear edge and are called ctenoid.
Pectoral finOne of a pair on the sides, just behind the gill cover. The paired fins steer, stop and hold the fish in place.
Pelvic finOne of a pair on the underside, also called the ventral fins.
Anal finA single fin on the underside, just behind the anus. With the dorsal fins it keeps the fish upright.
Caudal fin (tail fin)The main fin for driving the fish forward. It is the tail fin.
How this list was compiled: Every name checked on 2026-09-30 in two sources. R. W. Hegner, College Zoology (1912, public domain), the perch: fins, scales, operculum, gills, heart, the digestive tract from mouth to anus with its three pyloric caeca, liver, gall-bladder, air-bladder, kidneys and urinary bladder, https://archive.org/details/collegezoology00hegn. Florida Fish and Wildlife Conservation Commission, Fish anatomy: the outside and inside of a bony fish and what each part does, https://myfwc.com/fishing/freshwater/fishing-tips/anatomy/. Wikipedia, Fish anatomy: the organs of a yellow perch, pyloric caeca, swim bladder and gonads, https://en.wikipedia.org/wiki/Fish_anatomy. D. S. Jordan, A Guide to the Study of Fishes (1905, public domain), for the gall bladder, https://archive.org/details/guidetostudyoffi01jord. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work
How it works
This fish digestive system diagram shows a perch, a common bony fish, with its side opened. Food goes from the mouth through the stomach and intestine to the anus. A second view labels the outside. It is a plan, not to scale.
Read the full guide
Inside
The page opens on the inside with the names written in. Follow the food: esophagus, stomach, pyloric ceca, intestine, anus, with the liver and gallbladder beside them. High in the cavity lie the swim bladder and the kidney. The gills and the heart are at the front.
Outside
Switch to Outside for the fins, the gill cover, the scales and the lateral line. Press Quiz me and the names come off, so the page also works as a fish internal anatomy quiz.
A shark has no swim bladder and no gill cover. A bony fish like this one has both.
Questions and answers
What are the parts of a fish's digestive system?
In order: mouth, esophagus, stomach, pyloric ceca, intestine and anus. The liver makes bile, which is stored in the gallbladder and passed into the intestine.
What do the pyloric ceca do?
They are finger-like pouches that open into the intestine just past the stomach. They add surface for digesting and absorbing food. The number differs widely from species to species; a perch has three.
What does the swim bladder do?
It is a sac of gas that matches the weight of the fish to the water around it. The fish can then stay at one depth without effort.
Do all fish have a stomach?
No. Lungfishes and some other bony fish have none, and in them the esophagus opens straight into the intestine.