Humans have relatively generalized digestive systems, well suited to our omnivorous and varied diet. We must be able to digest a little bit of almost everything edible in our environment. Thus, it might be fair to consider the human digestive system fairly typical of the vertebrates. The same basic organs can be found in almost any fish, amphibian, reptile, bird or mammal, but often they are highly modified to accommodate a specialized diet or other ecological limits. Digestive systems provide a good example of how natural selection can modify a basic design to suit a variety of functions.
For a diagram of Amphioxus click on the button.It is worth considering the origins of the vertebrate gut before we consider its variations. We do not know exactly what the ancestral vertebrates were like, but there is evidence that they may have resembled a small creature called a sand lance, or amphioxus. Amphioxus has a fairly simple body plan. It feeds on detritus and bacteria that it sucks from among the sand grains along the ocean shore. It lacks any jaw structure and its mouth is a simple hole surrounded by a ring of tentacles used to set up currents that draw in water. Because its food is already in the form of tiny particles, it has little need for mechanical digestion. This was almost certainly true of all the earliest vertebrates.
The gut of amphioxus is a simple tube from mouth to anus, lacking the division into specialized organs that we see in humans. The only alteration to this tube is a pouch, the diverticulum, that branches off near the beginning of the tube. The walls of the tube and the diverticulum contain cells that secret digestive enzymes into the lumen. The relatively stagnant material in the diverticulum has time to be broken down by these enzymes. Absorption occurs by diffusion into the epithelial cells.
Two important changes occurred early in the history of the vertebrates. The first involved modification of outfoldings like the diverticulum to form accessory glands, such as the salivary glands, liver and pancreas. These have become specialized for manufacturing and secreting large quantities of digestive fluids, and they do not receive any food themselves.
The second major change has been the subdivision and specialization of the tube itself. When primitive vertebrates first developed a jaw, and began to take larger and more varied food, the need for more processing probably forced the specialization of stomach and intestines.
For a diagram of a shark's gut click on the button.Sharks are one of the oldest groups of vertebrates, and they tend to be very conservative in their body plans. It would be a mistake, however, to call them primitive. They are very well adapted for a carnivorous life in the ocean. The best known feature of sharks is, of course, their jaws. These are powerful, and lined with huge numbers of blade-like teeth. The teeth are actually derived from protective scales, but they have evolved as efficient slicing and tearing tools. A meat-eating shark can, in a single bite, take in hundreds of times what a similar filter feeder could get in the same time. The food, however, will require much more internal processing. The stomach of a shark must be large, expandable, powerful enough to mechanically break down a very large meal, and able to withstand the acid conditions required for bulk protein digestion.
Once food is released from the stomach, it is going to require huge supplies of protease enzymes to complete the digestion. Sharks have very large pancreases (green) compared to most other vertebrates to provide these enzymes.
Digestion of such large, bulk food requires substantial time, so a long intestine would benefit a shark, but here other aspects of ecology force a different strategy. Sharks depend on streamlining, which means the abdomen must narrow fairly rapidly. A shark with a pot-belly full of intestines would not be efficiently shaped for moving through the water. Instead, sharks have a short, thick, tapering intestine (blue), with a complex interior. A helical wall through the centre of the intestine creates a passage resembling a spiral staircase. The food passes slowly round and round the intestine as it moves toward the colon.
The colon is not required for resorption of water, since a shark is essentially isotonic with its environment. Specialized salt glands, which remove excess ocean salts from the blood dump their waste into the colon, where it can be eliminated.
For a diagram of a bird's digestive system click on the button.Birds represent the extreme of a tendency toward lightness and quickness seen also in some branches of reptiles. Flight has forced the reduction of weight in birds, leading to the loss of many organs. Birds lack teeth, having reduced the jaw substantially. A light, horny beak serves the purpose instead, but it lacks the ability to process food by chewing. Birds are largely either insect eaters or seed eaters. Their food tends to be small, so a tool useful for snapping at or plucking small food items is quite sufficient.
Small food items are likely to have very limited energy. To avoid having to feed continuously, a bird must store large quantities of food temporarily, releasing it for digestion as needed. The lower end of the oesophagus in many birds is modified as a thin-walled storage sac, the crop.
Both insects and seeds are quite hard, and difficult to digest. They must be broken down into smaller pieces to be exposed to enzymes. Lacking teeth, birds (and many reptiles) have used another solution. They swallow small stones which lodge in the muscular gizzard, the true stomach. The opening of the pyloric sphincter is very tiny, preventing these gizzard stones from escaping. As the gizzard churns, the stones grind against the food like numerous tiny millstones. Eventually, the stones are worn down so much they escape through the pyloric sphincter. Birds must constantly replace their gizzard stones by swallowing new gravel.
The rest of a bird's gut is much like our own, though chemically adjusted to suit the diet of the species.
For a diagram of a rabbit's gut click on the button.Most of the energy captured by plants is put into cellulose in cell walls. No animal has enzymes to digest cellulose, but any animal that could find a way to use this energy bonanza would have a significant advantage. In fact, many vertebrates appear to have independently evolved a similar strategy. In such animals as the rabbit, the horse and the koala, the stomach serves mainly as a storage sac for large quantities of vegetable matter. Relatively little digestion takes place in the small intestine, since there are no cellulose digesting enzymes there. As it leaves the small intestine, the food is diverted into a long dead end side branch, the caecum. The caecum houses a huge population of bacteria, some of which produce enzymes that convert cellulose to sugars, while others manufacture amino acids and other nutrients. Micromolecules are absorbed directly through the epithelium of the caecum, and the waste material is released into the colon for disposal. This is a relatively inefficient system, since much of the food is not available until it is too late to process it further. The supply of cellulose is so great, however, that these herbivores have thrived despite such inefficiency.
For a diagram of a cow's gut click on the button.A system that could process cellulose before food travelled through the main digestive organs would be more efficient. The ungulates - cows, deer sheep and their relatives - have found a way. The lower part of the oesophagus and the stomach of a cow are highly modified (the "four stomachs" of a cow). A large chamber, the rumen, is dedicated to storing grass and mixing it with a variety of symbiotic bacteria. This opens into a second chamber, the reticulum, with highly folded inner walls that provide a massive area for bacterial growth. The walls of this chamber are sold in grocery stores as "honeycomb tripe" The bacteria feed on the cellulose and grow to massive populations. Hours later, the cow will regurgitate a combination of caked bacteria and undigested food called cud. It will chew the cud and swallow it, this time diverting it to the third chamber, the omasum. Here water is withdrawn for recycling. From the omasum the cud enters the true stomach, or abomasum, where chemical digestion begins.
Because this all occurs before the food enters the stomach, digestive enzymes can now break down the various proteins, carbohydrates and lipids found in the bacteria. This extremely rich mixture of nutrients can sustain quite large animals on a diet mainly consisting of "indigestible" grass. In fact, a cow is not living directly on grass, but an a diet of bacteria. It is little wonder, with this special design for getting the best out of an abundant resource, that the wildebeest, bison, antelopes and caribou have become the dominant large mammals of their respective environments.