Animals have different types of alimentary tracts which vary in size, complexity, mode of feeding or type of diet. However, there is no definite alimentary canal in plants.
DIGESTION
Digestion is the breaking down of large molecules of food into simple and absorbable form for use by animals. In other words, digestion is the breaking down of food into simple, soluble and diffusible substances by mechanical and chemical means.
The digested food is absorbed into the fluids of the organism and transported to all the body cells where it is used up. Undigested food is egested from the body. Absorption of the digested food is similar in all organisms. The way an organism obtains and ingests food varies. Feeding mechanism depends on:
- whether an organism is holozoic, parasitic, or saprophytic; and
- the types of food the organism feeds on.
Holozoic organisms have special structures for capturing or obtaining their prey or food. These structures depend on whether the food is in a solid form or a fluid form. Solid food may be large or small.
Organisms feeding on large pieces of food or whole organisms use structures such as claws, tentacles, teeth and beaks to seize their food or prey. Those feeding on small pieces of food or tiny prey use filter-feeding mechanisms to obtain their food. Fluid-feeders have structures for sucking fluid food.
In most holozoic animals, digestion and absorption of food take place in the alimentary canal. A sample unicellular animal does not have alimentary canal. Digestion takes place inside the cell within a food vacuole. This is known as intracellular digestion.
Parasitic and saprophytic organisms take in food in fluid form. Saprophytes have special feeding mechanisms for changing solid food to liquid food before absorbing it. Many parasites have structures for boring into the bodies of their host and sucking the tissue fluids on which they feed.
PARTS OF ALIMENTARY TRACTS
A typical mammalian alimentary tract or canal includes the following parts: mouth, pharynx, esophagus(oesophagus), stomach, small intestine or ileum, cecum(caecum), appendix, large intestine or colon, rectum and anus.
However, there are modification of different parts of the alimentary tract in various animals. In some animals, some parts may be reduced or enlarged while in others, some parts may be absent.
In most higher animals, the alimentary canal is a long tube with two openings:
- the mouth at the anterior end for taking in food; and
- the anus at the posterior end for getting rid of undigested food.
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