Digestive System and Digestion

· The digestive system of earthworm consists of the tubular alimentary canal and digestive glands associated with it.
· Alimentary canal is a long straight tube with a variable diameter extending from mouth to anus. It consists of the following parts.
a. Mouth and Buccal cavity (1st to 3rd segments)
b. Pharynx (4th segment)
c. Oesophagus (5th to 7th segments)
d. Gizzard (8th segment)
e. Stomach (9th to 14th segments)
f. Intestine (15th to the last segment except for anus)
g. Anus (last segment)
· Digestion is the process of breaking down complex organic food by a biochemical process to change into a simple and soluble form to be utilized in the body.

1. Mouth and Buccal cavity
The mouth is a crescentic aperture lying below the fleshy lobe, the prostomium, in the peristomium. It leads into a short, narrow and thin-walled buccal cavity, extending up to the middle of the 3rd segment. The buccal cavity can be protruded out from the mouth for the ingestion of food particles.

Alimentary canal of the Earthworm

2. Pharynx
· Buccal cavity leads into a small, swollen, pear-shaped and thick-walled, muscular pharynx extending up to the 4th segment. The cavity of the pharynx is dorso-ventrally compressed. A large pharyngeal mass produces saliva. The lateral walls of the pharynx are internally folded which divide the pharynx into two horizontal shelves i.e. dorsal salivary chamber and ventral conducting chamber.
· The salivary chamber contains salivary glands formed by chromophil cells, which produce mucus [IOM 2069] and proteolytic enzymes.
· The ventral conducting chamber serves only as of the passage for the ingested food materials. So, the pharynx acts as sucking and pumping organ during feeding.
· In earthworms, the effective organ of food digestion is the pharynx [MOE].

3. Oesophagus
· Pharynx opens into a short, narrow, thin-walled tube extending up to 7th segments.

4. Gizzard
· It is an oval, hard, thick-walled and highly muscular organ lying in the 8th segment. 
· The muscular wall consists of circular muscles and is internally lined by thick, tough cuticles, which helps in grinding the food into a fine state [MOE 2069].

5. Stomach
· Gizzard is followed by the short, narrow, thin-walled, glandular and highly vascular tube extending from the 9th up to 14th segments.
· The glandular cells produce proteolytic enzymes and calciferous glands which secrete calcium and CO2. 
· A sphincter is present between gizzard and stomach and between stomach and intestine.

6. Intestine
· It is a long, wide, thin-walled tube from the 15th segment up to the last except the anus.
· It gives off a pair of small conical intestinal caeca in segment 26.
· Pre-typhlosolar region extends from the 15th to the 25th segment. It is highly glandular; contains intestinal caeca (one pair in 26th segment) [MOE 2012]
· It has a mid-dorsal fold, the typhlososle, that starts in segment 27 and ends at the 23rd or 25th segment in front of the anus.
· Typhlosole increases the absorptive surface [MOE 2062].
· The post-typhlosolar region is often called the rectum which contains faecal matter.

7. Anus
· It is a small, vertical slit-like (circular) opening situated in the last segment called the anal segment.
· It serves for throwing out undigested food particles and mud in the form of castings.

8. Digestion
· Digestion of food in various parts of the alimentary canal is shown in the following table.

Table: Digestion of food in Earthworm
Part of the alimentary canalSecretionDigestionEnd product
Buccal chamber---
PharynxSaliva contains mucus and proteasea. mucin lubricates the food
b. Protease causes hydrolysis of protein
GizzardGrinding of food--
(Calciferous glands)
Secretion of stomach-Neutralizes the humic acid of soil
Intestinal gland cellsa. Pepsin
b. Trypsin
c. Amylase
d. Lipase
e. Cellulase
f. Chitinase
Amino acids
Fatty acids Glycerol

· Absorptive cells of the intestine absorb the end products of the digestion and transfer them to the blood capillaries of the intestine for the distribution to all parts of the body. For this, typhlosole is the region of the greatest digestion and absorption.
· Villi also aid in absorption.
· The undigested food matter along with the soil is passed out through the anus in the form of little heaps or pellets called the worm castings.


· Earthworm lacks specialized breathing devices and depends on cutaneous (skin) respiration.
· Skin is remarkably adapted for this and it requires humid conditions to prevent the drying out.
· Gaseous exchange i.e. intake of oxygen and giving out of Carbon dioxide takes place between blood capillaries of an outer epidermis and surface film of moisture contributed by secreted mucus, excreted wastes and coelomic fluid.
· Haemoglobin is dissolved in the plasma of blood and it acts as a respiratory pigment.

Also, Read our Other Notes for Entrance Exams:

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