"Goldfish don't have a stomach and so they can't digest food like you or I. It takes about 5 mintues for their food to come out the other end."
Quoted from this website, Koko's Goldfish World: http://www.kokosgoldfish.com/care.html
The purpose of the stomach is to digest food into an easily absorbable liquid. Actual absorption of most nutrients takes place in the intestines.
Incidentally, I once knew a man who had his stomach removed during WWII, due to a very serious illness. He had very specific dietary and medical requirements, but for the most part lived a fairly healthy, if somewhat inactive life. And much like a goldfish, food typically went through him in a matter of minutes.
Disagree if you must, but biology -- specifically, biomedical technology -- was my major in college. As well, I owned goldfish for many years.
MONOGASTRIC DIGESTIVE SYSTEM e.g. the pig
Mouth - teeth are used to reduce sizes of food particles, salivary glands to provide lubrication and aid swallowing. Saliva also contains an enzyme amylase that digests starch to sugar (evolutionary function to help identify useful and avoid dangerous foods?). Food then passes down esophagus into
Stomach - a hydrochloric acid bath. The acid kills bacteria and aids the digestion of food. The stomach has a unique mucous layer to protect it from self digestion by the acid. The stomach also contains a proteinase (pepsin) which begins digestion of proteins to polypeptides etc. Leaves stomach and enters duodenum. In young mammals another enzyme, rennin, is also present which aids clotting and digestion of milk proteins.
Duodenum - first part of small intestine into which pancreatic juice and bile are secreted. Bile helps emulsify fat (break it into smaller droplets) making its digestion easier. Pancreatic juices contains several enzymes trypsin, chymotrypsin which acts on proteins and polypeptides, amylase which breaks down starch, steapsin which breaks down fats and collagenase which digests collagen.
Small intestine - the lining is covered with small finger-like protuberances or villi which increase the surface area for nutrient absorbtion.
Enzymes secreted by the intestine wall include peptidase, sucrase and maltase. Molecules of the food nutrients such as glucose, amino acids, fatty acids etc can be absorbed here and passed into blood system.
Large intestine (cecum and colon) - large populations of bacteria are present which provide some cellulase digesting activity. The cecum is a blind ending sac which is very small in most monogastrics. In humans it is referred to as the appendix. The cecum contains microorganisms which perform some digestion of plant material. The major function of the large intestine is water absorption. The microorganisms present also manufacture some essential vitamins (B vitamins and vitamin K) for use by the host animal.
Food has what is described as a moderate speed of passage through the gut, usually 24-48hrs