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What is digestion? Give an account on the digestive tract of Pheretima with suitable diagram.

Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma.

The digestive system of Pheretima consists of the alimentary canal and the digestive glands.

Alimentary canal : The alimentary canal runs as a straight tube throughout the length of the body from the mouth to the anus. It consists of following regions : 

  1. Mouth and Buccal Cavity : The mouth is situated in the first segment called peristomium.                              The mouth leads into a short, thin walled tube called buccal chamber or buccal cavity running up to third segment.
  2.  Pharynx : It is situated in third and fourth segment. It is a pear-shaped, broad and muscular structure separated from the buccal cavity by a groove. Its lateral walls internally form two horizontal folds or shelves. Chromophill cell is a cilia containing special cell in dorsal chamber which produces mucin to lubricate food materials. Protease enzyme is produced in salivary chamber. Food passes to oesophagus from pharynx.
  3. Oesophagus : Behind the pharynx is a short, narrow and thin-walled oesophagus.
  4. Gizzard : It is a grinding (pulverising) organ of Pheretima present in 8-9 segment. There is presence of circular muscle only in muscularis which helps in grinding. Well developed inner cuticle breaks down food materials. Digestive enzymes are absent.
  5. Stomach : Digestion of protein occurs here. Calciferous glands present in enteric epithelium help in neutralization of humic acid.

6. Intestine : It is the main site of digestion and absorption of food materials.

Typhlosole is highly glandular and vascular villi like longitudinal ridge present in intestine. It is richly supplied by blood vessels. It increases surface area for absorption.

  • Pre-typhlosolar region : It is present in 15-26 segments.Anteriorly directed Intestinal caecum present in 26th segment helps in digestion of cellulose. 
  • Typhlosolar region : It is present in 27 to last segment except 23-25 segments before anus.
  • Post-typhlosolar region : Post typhlosolar region is present in 23-25 segments in front of anus. It is the third region of intestine and is called the rectum. Undigested food is stored temporarily here.

7. Anus : It lies at last segment. Undigested food material is egested in the form of Earthworm casting.