Earthworm Digestive System in Detail with Diagram

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An earthworm’s Digestive System is quite similar to that of higher animals.

It ingests food by the pumping action of its pharynx.

The food is engulfed by the rhythmic contractions of the pharyngeal wall, which further leads to the buccal chamber.

EARTHWORM DIGESTIVE SYSTEM
Credit:naturewatch.ca

The Earthworm digestive system

The earthworm digestive system comprises the following parts:

Alimentary canal

The alimentary canal in earthworms is seen as complete, which runs along the entire length of the body from the anterior mouth to the posterior anus.

The alimentary canal can be subdivided into various parts, which are:

  • Buccal chamber
  • Pharynx
  • Esophagus
  • Gizzard
  • Stomach
  • Intestine

Buccal chamber

  • The crescent-shaped mouth is present at the anterior end of the peristomium and towards the ventral side of the prostomium.
  • This further leads to a protrusible, short, narrow buccal chamber extending up to the middle of the third segment of the earthworm body.

Pharynx

  • The pharynx in earthworms is pear-shaped and muscular, extending up to the fourth segment of the body.
  • The buccal chamber dilates into the pharynx.
  • The anterior end appears like a nerve ring placed in a transverse groove between the pharynx and the buccal chamber.
  • The cavity of the pharynx is somewhat flattened due to the presence of a large glandular pharyngeal mass on the roof of the pharynx.
  • This mass secretes the salivary secretions for mixing with food.
  • The horizontal shelf is formed on the lateral walls of the pharynx.
  • It distinguishes the pharyngeal cavity into the dorsal or salivary chamber and a ventral or conducting chamber.
  • Salivary secretions comprise mucus and proteolytic enzymes, which help digestion. This secretion occurs in the salivary chamber.

Esophagus

  • The esophagus lies behind the pharynx, which is also called the gullet.
  • It is a short, narrow, thin-walled tube extending to the seventh segment.

Gizzard

  • The gizzard is an oval, hard, and prominent structure that lies in the eighth segment of the body of an earthworm.
  • It is the modification of the esophagus, a thick-walled muscular organ consisting of muscle fibers lined by a tough cuticle.

Stomach

  • The gizzard ends up in the narrow tube called the stomach.
  • This stomach extends from the 9th segment to the 14th segment of the earthworm.
  • A sphincter is present at every end of the segments lying in the stomach.
  • The stomach walls are highly vascular and glandular and have transverse folds.

Intestine

  • The part next to the stomach is called the intestine.
  • It is a long, wide, and thin-walled tube extending from the 15th segment to the last segment.
  • The intestine has a beaded appearance because of the constrictions present at the septa.
  • The internal lining is ciliated, folded, vascular, and glandular.

The intestine can be divided into the following:

Pre-typhlosole region

  • The pre-typhlosole region runs between the 15th and 26th segments in the anterior portion of the earthworm.
  • The walls of this region have numerous folds or minute processes called villi and have a rich supply of blood vessels.
  • From the 26th segment, a pair of forwardly-directed lateral conical outgrowth is seen.
  • Intestinal caeca run-up to 22nd and 23rd segments.
  • This intestinal caecum has the lining of vascular vessels where Villi-like processes are seen.

Typhlosolar region

  • This is the second part, or the middle part of the intestine.
  • It lies between the 27th segments up to 23-25 segments in front of the anus.
  • This part is characterized by glandular, vascular longitudinal ridges that are called typhlosole.

Post-typhlosole region

  • This is the last part of the intestine and is called the rectum.
  • It extends for about 23-25 segments and opens outside through a terminal anus.
  • This region has longitudinal folds

Physiology of digestion in Earthworm Digestive System

  • Ingested food moves posteriorly in the earthworm.
  • No digestion takes occurs inside the buccal chamber.
  • As the food passes through the pharynx, it meets with the salivary secretions of the pharyngeal mass and pours into the pharyngeal chamber for mixing up.
  • The secretion contains mucin, which lubricates the food, and an enzyme protease acts on the food to digest protein.
  • Food now makes its way to the gizzard through the stomach.
  • The gizzard acts as a grinding machine and helps grind and further pulverize the food masses.
  • In the stomach, a chalky substance is secreted from the calciferous glands of the stomach wall that neutralizes the humic acid present in the soil that is devoured along with food.
  • The intestine is the main organ for the digestion of food, where the intestinal walls secrete digestive juices containing pepsin, trypsin, amylase, lipase, and cellulase.
  • The digestion is extracellular in the earthworm.
  • The intestines absorb most of the digested food required by the earthworm and pass out the indigested food out of the body through the anus.
  • The intestinal wall has typhlosoles that are similar to human villi. These pipe or channel-like structures increase the surface area of absorption.

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