Animal nutrition includes nutrient requirement, mode of intake of food and its utilisation in the body.
The breakdown of complex components of food into simpler substances is called digestion.
The mode of taking food into the body varies in different organisms. Bees and humming-birds suck the nectar of plants, infants of human and many other animals feed on mother's milk. Snakes like the python swallow the animals they prey upon etc.
The digestive system has various steps in human body. The canal can be divided into various compartments: (1) the buccal cavity, (2) foodpipe or oesophagus, (3) stomach, (4) small intestine, (5) large intestine ending in the rectum and (6) the anus. Is it not a very long path? These parts together form the alimentary canal (digestive tract).
The food components gradually get digested as food travels through the various compartments.
The digestive tract and the associated glands together constitute the digestive system.
The mouth and buccal cavity
Food is taken into the body through the mouth. The process of taking food into the body is called ingestion. We chew the food with the teeth and break it down mechanically into small pieces.
Our mouth has the salivary glands which secrete saliva. The saliva breaks down the starch into sugars.
The tongue is a fleshy muscular organ attached at the back to the floor of the buccal cavity. It is free at the front and can be moved in all directions. 
The foodpipe/oesophagus
The swallowed food passes into the foodpipe or oesophagus. The foodpipe runs along the neck and the chest. Food is pushed down by movement of the wall of the foodpipe.
Actually this movement takes place throughout the alimentary canal and pushes the food downwards.
At sometimes the food is not accepted by our stomach and is vomited out.
The stomach
The stomach is a thick-walled bag. Its shape is like a flattened J and it is the widest part of the alimentary canal. It receives food from the food pipe at one end and opens into the small intestine at the other.
The inner lining of the stomach secretes mucous, hydrochloric acid and digestive juices. The mucous protects the lining of the stomach.
The digestive juices break down the proteins into simpler substances.
The small intestine
The small intestine is highly coiled and is about 7.5 metres long. It receives secretions from the liver and the pancreas.
The liver is a reddish brown gland situated in the upper part of the abdomen on the right side. It is the largest gland in the body. It secretes bile juice that is stored in a sac called the gall bladder.
The bile plays an important role in the digestion of fats. The pancreas is a large cream coloured gland located just below the stomach.
Absorption in the small intestine
When the digested food can now pass into the blood vessels in the wall of the intestine, the process is known as absorption. The inner walls of the small intestine have thousands of finger-like outgrowths is called villi.
The surface of the villi absorbs the digested food materials. The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by the body is called assimilation. In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water, and energy is released. The food that remains undigested and unabsorbed enters into the large intestine.
Large intestine
The large intestine is wider and shorter than small intestine. It is about 1.5 metre in length. Its function is to absorb water and some salts from the undigested food material. The remaining waste passes into the rectum and remains there as semi-solid faeces. The faecal matter is removed through the anus from time-to-time is called egestion.
When we observe the cows, buffaloes and other grass-eating animals chewing continuously even when they are not eating. Actually, they quickly swallow the grass and store it in a part of the stomach called rumen.
When the food partially digested and is called cud. But later the cud returns to the mouth in small lumps and the animal chews and the process is known as rumination and these animals are called ruminants.
The grass is rich in cellulose, a type of carbohydrate. In ruminants like cattle, deer, etc., bacteria present in rumen helps in digestion of cellulose. Many animals, including humans, cannot digest cellulose.
Amoeba is a microscopic single-celled organism found in pond water. Amoeba has a cell membrane, a rounded, dense nucleus and many small bubble-like vacuoles Amoeba constantly changes its shape and position. It pushes out one, or more finger-like projections, called pseudopodia or false feet for movement and capture of food.
Amoeba feeds on some microscopic organisms. When it senses food, it pushes out pseudopodia around the food particle and engulfs it. The food becomes trapped in a food vacuole.