According to C. M. Yonge the alimentary canal in animals has the following important functional regions: 1. The region of reception 2. The region of conduction and storage 3. The region of internal trituration and digestion 4. The region of final digestion and absorption 5. The region of faeces formation.
1. The region of reception:
This region includes the mouth and its associated appendages and cavity. This region is primarily meant for food selection and mechanical break-down of the food. Food selection is made by taste, smell and texture.
The mouth cavity often contains salivary glands which secrete enzymes in few mammals such as man but in rest cases secrete copious mucus which is by its nature known as lubricating fluid.
Blood sucking animals such as mosquitoes, usually have an anticoagulant in their salivary secretion which prevent the blood from clotting, while carnivores feeding on live prey may secrete paralyzing toxins.
Many insects and vertebrates secrete carbohydrate splitting enzymes, while some carnivores and cephalopods secrete protein-splitting enzymes, in addition to poison and mucus.
2. The region of conduction and storage:
This includes the oesophagus and the crop of some animals. They are often muscular and are meant for storing up the food as in case of leeches and conduction as well.
3. The region of internal trituration and digestion:
This region includes the gizzard and the stomach of some animals. They are modified in different animals in order to carry out the internal trituration and digestion of the ingested food.
Digestive enzymes may be secreted into the stomach by unicellular gastric glands situated in the lining or by glandular diverticula or caeca which help in digesting the food contents.
4. The region of final digestion and absorption:
This includes the small intestine where the final digestion and absorption of the food materials take place. Digestion of the food materials takes place with the help of enzymes which are either liberated in the preceeding region as in most insects or in the same region as in most vertebrates.
5. The region of faeces formation:
It is the last region of the alimentary canal where the absorption of water from the undigested food takes place and subsequently undigested food materials are twisted together with mucus into faeces.
This region is very conspicuous in terrestrial animal as in insect hindgut and vertebrate colon.