Different species of animals have different digestive systems which are adapted to their unique requirements. The type of food, method of food gathering and energy needs are some factors that influence the type of digestive system an animal needs in order to survive.

Herbivores have a more specialised digestive system than that of a carnivore because it is more difficult to digest vegetation than meat. The teeth are flat so that grass and plant material can be ground down, rather than the sharp teeth of carnivores designed to tear flesh.

Animals which eat both plants and meat, such as humans, have both types of teeth so that they can perform both functions. In simple animals the digestive system is not complex, usually containing a single tube. As the animal becomes more complex organs with specialised functions develop. Higher order animals require a storage organ, such as the stomach, which allows them to take in large amounts of food in one feeding and then use its energy over a long period of times. This makes it possible for them to devote time to activities other than feeding.

Birds need a high body temperature which requires a large amount of energy to maintain. This means that birds need to eat larger amounts of food to gain the energy they need every day to survive.

In order to do this they have a specialised digestive system where there is an efficient absorption of energy. Food passes through very quickly and is all absorbed, leaving little waste. Birds have no teeth so digestion does not begin in the mouth, all of the food breakdown must occur within the digestive system. Food enters through the mouth where it passes down the esophagus into the crop. This organ is where the food is stored and begins to soften. From here it moves into the stomach, which is called the proventriculus. This acts as a true stomach where digestive juices continue to chemically break down food. The partially digested food moves into the muscular gizzard, which has a rough lining to break down the food further. It sometimes contains sand or pebbles which have been swallowed by the bird, which add to the grinding process.

The food moves into the intestine, first into the small intestine and then onto the large intestine. At the point where the small and large intestine meet are two pouches or caeca, which absorb the water from the food. In herbivores this is the site of cellulose deposition. The food becomes harder and enters into a chamber called the cloaca. It then passes out of the body through the cloacal lining.

An adult horse, over five years old, has 40-42 teeth which include incisors, canines and cheek teeth. The incisors work together with the lips to grasp and move food around the mouth. The molars are used to grind down food, making it easier to digest. The digestive system has developed to effectively break down and digest fodder, with the stomach being smaller than that of other cud chewing animals.

The average adult horse is able to hold 7.5-9.5 litres (2-2.5 gallons) of chyme in their stomach. The small intestine is 18-21 metres (60-70 feet) long, while the large intestine is enormous, adapted to digesting grass and hey. The caecum is a pocket between the large and small intestine and is able to hold 15-65 litres (4-17 gallons) while the large colon can hold 60-150 litres (16-34 gallons). In the large intestine food is broken down through fermentation by both bacteria and protozoa. Food may remain in the intestine for up to 55 hours.

The digestive system in insects is basically a tube that begins at the mouth and ends at the rectum. It can be divided into a pharynx, esophagus, stomach, intestine, colon and rectum. The stomach or midgut has glands called the gastric ceca which secrete the digestive juices. The malpighian tubes removes the nitrogen rich waste from the blood. Each tube empties at the connection between the stomach and large intestine, producing an end product of uric acid, which is passed with faeces.

Insects such as the praying mantis are carnivorous and begin to digest their food by chewing. Other insects such as wasps paralyse their pray and lay their eggs in the bodies. This provides a living food supply for the young, providing their immediate source of food. Termites gain their energy from wood and begin to digest the cellulose before it enters the digestive system. Protozoans are released onto the wood and begin its breakdown, continuing to act on the wood once it is passed onto the stomach. Flatworms have a simple digestive system, with a single tube serving as both mouth and anus.

The digestive system of a crustacean varies depending on the species. In simple species it is a single tube, while others have a specialised system with chambers and organs, each with a specialised function. This often reflects their feeding habits, some animals are scavengers and their food is often beginning to decay. Others require more complex systems as they fully digest the food.

Snakes eat all parts of their pray and need a specialised digestive system to gain the most nutrients from their food. Their teeth are very thin and usually curve backwards. Their function is not to grind down food as it is in most animals, rather it is to capture prey. The food is swallowed whole, thus the teeth perform a specialised function. They have powerful digestive enzymes to break down the hair, feathers, bones, organs and other parts of their food.

The salivary glands also produce strong enzymes which are also used to kill the organism. If saliva enters the wounds of the animal it will begin the digestive process and cause severe tissue damage, which can often lead to the death of the animal. These toxic substances are found in the saliva of many non poisonous snakes. In poisonous snakes it is the salivary glands which have developed into venom sacks, with the venom being a highly toxic form of saliva.

The mollusk digestive system has millions of microscopic hair like fibres along the main digestive tract and has several divisions for the different organs.

The first section contains the mouth and esophagus and is the site of the initial breakdown of food. There is a specialised filelike radual found in the mouth, which acts like teeth or a tongue in the food breakdown.

Oysters, clams and muscles do not feature the radula, as they are filter feeders, the food is already filtered when it enters the mouth and continues down the digestive tract. It reaches the liver and stomach, which continues the digestion. In many mollusks the stomach has a flexible rod, which is made up of mucus and proteins in a crystalline structure. This secretes the digestive juices and enzymes and acts as a kind of stirring stick, mixing up the stomach contents to aid digestion. The final section of the digestive tract contains the intestine and anus, from which the waste is removed.

Bibliography

Encyclopaedia Britannica, 15th Edition. (Chicago: University of Chicago, 1986)

The Book of Popular Science Encyclopaedia. (New York: Grolier, 1961)

The Software Toolworks Multimedia Encyclopaedia, Release 6. (New York: Grolier, 1996)