Alimentary Canal

Alimentary Canal Definition

The alimentary canal is a continuous passage starting from the mouth and ending at the anus, which carries food through different parts of the digestive system and allows waste to exit the body. The alimentary canal varies widely in organism, but is only seen in organism which are bilaterally symmetrical. Various sections of the alimentary canal contain cells which secrete digestive enzymes, allowing food to be broken down. Other specialized cells allow for the absorption of materials into the body. In human and other highly complex animals, the alimentary canal is organized into specialized tissues and organs. These organs and tissues were derived from the alimentary canal of our earliest ancestors, which likely consisted of a simple tube connecting the mouth and anus.

Alimentary Canal Organs

The organs present in the alimentary canal vary widely between groups of organisms. Some organisms have no well-defined organs or tissues in their alimentary canal, while other have many unique structures. Starting from the mouth, a membrane lined tube connects the mouth to the esophagus, which is called the pharynx. The pharynx has evolved a number of functions in different animals, from housing the gills to providing a structure for filter feeding. Typically, the alimentary canal then continues through the esophagus, which carries food to the stomach. Some animals, such as ruminants, have multiple stomachs which carry different enzymes and microbiomes to process different parts of their food. After the stomach, food typically passes into the small intestine, which is responsible for extracting the newly freed nutrients into the body, as well as continuing the breakdown of foods. The configuration and arrangement of the small intestine can vary widely, but it usually ends by dumping its contents into the large intestine. The large intestine functions within the alimentary canal to remove excess water and any remaining nutrients from the food being processed. By the end of the large intestine, only waste and indigestible material remains, and is excreted as stool. The alimentary canal ends at the anus, where waste is excreted into the environment.

Other groups of animals, such as birds, have an entirely different arrangement of organs in the alimentary canal, and contain structures not seen in humans. For example, birds often have a gizzard, which is a muscular organ used to grind food before it enters the stomach. Mammals do not need this organ because they have the ability to masticate, or chew their food. Other adaptations to the alimentary canal include glands that secrete digestive substances, toxic fangs and teeth, and specially adapted intestines which help animals digest the food available in their niche.

Alimentary Canal Layers

Within the bilaterally symmetrical animals, there are two groups: the protostomes and the deuterostomes. Both groups develop an alimentary canal with three tissue layers, but through different methods. Both groups develop from a single celled zygote into a sphere of cells known as the blastula. During embryonic development, parts of the sphere will fold inward, and connect to the other side, forming the alimentary canal. When this happens, the layers of the ectoderm are separated from the endoderm. The ectoderm will become the outer layers of the body, while the endoderm will define the lining of the alimentary canal. The organs that operate with the alimentary canal, such as digestive glands and organs like the liver, are formed from either the endoderm, or from a third tissue that lies between the entoderm and endoderm, the mesoderm.

Related Biology Terms

  • Esophagus – A tube, derived from the endoderm of the embryo, which carries food through the thoracic cavity to the stomach below.
  • Intestines – Part of the alimentary canal which extracts nutrients and water from the material inside.
  • Pharynx – A special tube which connects the mouth to the esophagus.
  • Coelom – A cavity within the body, which helps separate the organs and circulatory systems, but never opens to the external environment.

Quiz

1. The gall bladder is a small sac that is attached to the underside of the liver. A small tube connects the bladder to the intestines, where it releases bile, a substance which helps dissolve many substances. Is the gall bladder part of the alimentary canal?
A. Yes
B. No
C. Only in animals that have one

Answer to Question #1
B is correct. The gall bladder, and other organs which attach to the alimentary canal are not typically considered a part of it. The alimentary canal, by definition, carries ingested food through the body. The other organs which are considered part of the alimentary canal include the stomach and intestines, both of which food passes directly through. Many other tissues and glands are associated with the alimentary canal, but do not participate directly in the passage of food.

2. A special type of muscle fiber, known as smooth muscle, surrounds almost all the organs of the alimentary canal. These muscles cannot be contracted voluntarily, like skeletal muscle. What is the purpose of these muscles?
A. To move food through the alimentary canal
B. To protect the canal from movements of the body
C. To change the shape of the organs when the organism needs to squeeze through a space

Answer to Question #2
A is correct. The purpose of the smooth muscle in the alimentary canal is to produce contractions that squeeze food mater through the different parts of the digestive system. Voluntary muscles in your throat can start these contractions when you swallow, but after the food passes into your esophagus, the process becomes unconscious. Like squeezing out a tube of toothpaste, these muscles contract behind a food mass to move it forward. The skeletal muscles of the abdomen are usually the ones that do the moving and squeezing of the entire body, and can be controlled voluntarily. Because the digestive system is mostly soft tissue, it isn’t easily damaged by the movements of an organism.

3. A jellyfish is an animal that only has two layers, instead of one. When the blastula folds inward, it creates a mouth, but never forms an anus on the other side of the organism. Instead, waste products are simply releases out of the mouth. Does the jellyfish have an alimentary canal?
A. Yes, it is just “U” shaped
B. No, it must connect to an anus
C. Yes, it just is not as well-developed as some

Answer to Question #3
B is correct. An alimentary canal requires that the mouth lead to an anus. Many organisms like the jellyfish do not have an alimentary canal. Even starfish, which evolved from forms that probably had an alimentary canal, have lost the characteristic in favor of a more efficient “open” digestive system. There are organisms which have a “U” shaped gut, and they typically live in a shell or under the sand and need to deposit their waste back at the surface. This can be seen in many worms and mollusks.
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