Digestive System of Earthworm

The digestive system of the earthworm is a specialized and linear system designed for the breakdown and absorption of organic material from the soil. It plays a vital role not only in the earthworm’s nutrition but also in soil fertility by aiding in the decomposition of organic matter and recycling nutrients.

1.0Introduction

• Systematic Position
• Phylum: Annelida
• Class: Oligochaeta
• Genus:Pheretima
• Species: Posthuma

2.0Habit and Habitat

• These are nocturnal in habit and live in damp, moist, humus-rich soil of lawns, gardens, etc. 
• In dry weather, they burrow deeper into the soil to avoid dryness. 
• Their niche is herbivore and macro-decomposer. 
• It is important as a source of food for birds and helps in soil aeration and increasing soil fertility.

3.0Digestive System of Earthworm

The digestive system of earthworms consists of an alimentary canal and glands, along with the physiology of digestion.

1. Alimentary Canal

• The alimentary canal is long and straight, extending from mouth to anus. It consists of the following parts:
• Mouth : 1stsegment
• BuccalCavity : 2nd-3rdsegment or middle of 3rdsegment
• Pharynx: 3rd-4th segment
• Oesophagus: 5th-segment
• Gizzard : 8th or 8th-segment
• Stomach : 9th or 10th-14th segment
• Intestine : 15th up to last segment except anus 
• Anus : last segment

2. Mouth

• MouthIt is a crescentic aperture situated in the 1 segment below the prostomium.
• Mouth leads into a buccalcavity. Ingestion of food takes place through it.

3. Buccal Cavity

• It is a short, wider, thin-walled tube extending from 2nd up to 3rd or middle of 3rd segment. 
• It consists of two kinds of muscle. 
• They are the protractile and retractile muscles. 
• Buccalavity protrudes out through the mouth with the help of a special muscle that holds food particles during feeding.
• The buccal cavity leads to a spacious organ called the pharynx.

4. Pharynx

• It is a small, swollen, wider, thick-walled pear-shaped chamber that extends up to the fourth segment. It is wider than the buccal cavity.
• It is distinguished from the buccal cavity by means of constriction. The pharyngeal gland is located in the dorsal salivary chamber. 
• The pharyngeal gland comprises many chromaphil cells, which produce saliva containing proteolytic enzymes; protease and mucin. 
• Mucin makes the food soft, and protease converts protein into amino acids.

5. Oesophagus

• It is a narrow, thin-walled tubular structure extending from the 5th to the 7th segment.
• It has no gland. 
• It passes food particles from the pharynx to the gizzard and leads into a gizzard.

6. Gizzard

• It is an oval, thick-walled and highly muscular organ in the 8th or 8th-9th segment.
• It is the hardest part of the alimentary canal because the inner lining of the cuticle is present. 
• It also possesses chitinous teeth, like projection. These teeth help in grinding or crushing food, so they act as grinders during feeding.

7. Stomach

• The gizzard leads to a short, narrow, thin-walled, highly vascular tubular structure called the stomach, which extends from the 9th or 10th to the 14th segment. 
• It is wider than the esophagus. It has a calciferous gland, which helps neutralize food through the calcification process. The stomach leads to the intestine.
• The glandular cell of the stomach produces proteolytic enzymes for the digestion of protein.

8. Intestine

• It is a long, wide and thin-walled tube which extends from the 15th to the last segment, except the anus. Its inner lining is ciliated, vascular, folded and glandular. 
• Its intestinal lining is folded to form villi. One villi becomes larger and more developed than the other, called typhlosole, which runs mid-dorsally from the 27th to the last 25th segment.
• Typhlosole divides the intestine into 3 regions. They are:

i. Pre-typhlosolarregion

• It extends from the 15th segment to the 26th segment, so it is the first part of the intestine.
• It consists of villi but no typhlosole. 
• In the 26th segment, a pair of short and conical lateral outgrowth called intestinal caeca extends upward up to the 23rd segment. 
• Intestinal caecaproduce amylase which helps in starch(carbohydrate) digestion.

ii. Typhlosolarregion

• The second or middle part of the intestine extends from the 27th segment to the 25th segment. It has villi and typhlosole.
• The typhlosoleis is a highly vascular and glandular fold that increases the absorptive surface area of the intestine.

iii. Post-typhlosolarregion

• It is the last part of the intestine lying in the last 23rd-25th segment in front of the anus. 
• It is also called rectum. It lacks villi and typhlosole.
• It contains small pellets of mud, which are thrown out through the anus to form casting.

9. Anus

• It is a circular opening in the last segment called the anal segment.
• Undigested food materials release through the anus in the form of worm casting.

4.0Digestive Glands

• Different types of digestive glands are associated with the alimentary canal of earthworms.
• Pharyngeal gland
• Gastric gland
• Intestinal glands
• Intestinal caeca

5.0Physiology Of Digestion

• Digestion is the bio-chemical process in which complex organic food is broken down into
• simple, soluble and diffusible form in the presence of respective enzymes.
• Earthworms feed upon all kinds of organic humus and debris, such as decaying leaves and
• seeds, protozoan, etc., present in the soil. 
• They also feed directly on leaves, grasses and other vegetation.
• During feeding, the buccal cavity is protruded out with the help of protractile and retractile muscles. Then, the food is drawn into the mouth. 
• The ingested food enters the pharynx through the buccal cavity. 
• The dorsal chamber of the pharynx consists of a pharyngeal gland composed of chromophil cells that produce saliva containing mucin and protease. 
• Mucinlubricates the food, and protease converts protein into amino acids. 
• The foods then pass through the esophagus into the gizzard, where the grinding or crushing of food material takes place in a fine state due to the contraction of the circular muscles of the gizzard. 
• The ground food material enters the stomach, where food neutralization occurs through the calcification process. 
• Also, protein is completely digested through proteolytic enzymes. The food material then enters the intestine. 

In the intestine, intestinal care produces amylase which converts starch into glucose. In the intestine, several enzymes are secreted and act on the substrate as follows:

• Protease: converts protein into amino acid.
• Amylase: converts starch into two molecules of glucose i.e. maltose.
• Cellulase: converts cellulose into glucose
• Chitinase: digest chitin of the exoskeleton of insects.
• Lipase: converts fats into fatty acids and glycerol.
• Digestion occurs mostly in the intestine, and villi absorb the digested food. Then, pass into the bloodstream through capillaries. Undigested food and soil are released in the form of casting through the anus.