Digestive System of Earthworm: Diagrams, FAQs & MCQs for Zoology Students

 

Digestive system of Earthworm: Alimentary canal, Physiology of digestion, related important questions and MCQs

            

The digestive system of earthworms comprises the alimentary canal which is a straight tube running along the entire length of the body.

The anterior end opens as the mouth and posteriorly it ends up as an opening via the anus.

Digestive System of Earthworm Diagram 

Alimentary canal

The Alimentary canal is divided functionally into different parts which are:

 Mouth 

A crescentic mouth is situated at the anterior end of the peristomium on the ventral side of the prostomium.

 Buccal chamber

The mouth leads to a short and narrow protrusible buccal chamber which reaches up to the middle of 3rd segment. Anteriorly the epithelial lining shows longitudinal folds. 

Diagram: T S. through pharynx, gizzard and intestine of Earthworm 

* Note: The above diagram is provided to make it easier to compare and understand the physiology by observing the transverse section (T.S.)of the body of Earthworms passing through different regions.

Pharynx 

The buccal chamber continues as a pear-shaped muscular pharynx, extending up to the 4th segment.

Between the buccal chamber and pharynx, a transverse groove holds a nerve ring.

A large glandular pharyngeal mass producing salivary secretions (containing mucus and proteolytic enzymes), compresses the pharynx such that the lateral walls of the pharynx are pushed inside forming a narrow horizontal shelf on each side.

The shelves anteriorly meet to form a salivary or dorsal chamber and posteriorly meet to form a conducting or ventral chamber. Salivary secretions are poured into the salivary chamber.

Oesophagus (or gullet)

The oesophagus runs from the 4th to the 7th segment. It is a short narrow thin-walled tube.

Gizzard

In the 8th segment lies a prominent, oval, hard and thick-walled muscular organ, the gizzard, consisting of circular muscle fibres and internally lined by tough cuticles. It is the most muscular part of the alimentary canal of earthworms.

 Stomach

The gizzard is followed by a short narrow tube, the stomach, extending from the 9th to 14 segments, having a sphincter at each end. The wall of the stomach is highly vascular and glandular and thrown into internal transverse folds.

Intestine

The longest part of the alimentary canal of earthworms is the intestine which extends from the 15 segment to the last segment, behind the stomach. It is a wide and thin-walled tube. It has a beaded appearance due to circular constrictions corresponding to the septa. Internally it is ciliated, folded, vascular and glandular.

The intestine is divided into three parts:

Pre-typhlosolar region

It is the anterior part of the intestine between segments 15 to 26. It is highly vascular and its wall is internally folded to form the villi.

From the 26th segment, a pair of forwardly directed lateral conical outgrowths called the intestinal caeca are given out which run up to the 22nd or 23rd segment. They are highly vascular and show the presence of villi-like processes internally.

 Typhlosolar region:

The region starting from 27 segments up to 23-25 segments before the anus, is characterized by the presence of a highly glandular and vascular longitudinal ridge on the median of the dorsal side of the intestinal cavity and is called the typhlosole.

 Post-typhlosolar region:

It is the third or the last part, also called the rectum and is 23 to 25 segments long. Internally it shows longitudinal folds and opens outside through the terminal anus.

Related Important Topics:

 

Physiology of Digestion:

Ingested food is moved posteriorly by pressing action. No digestion occurs in the buccal chamber.

In the pharynx, when the food passes through the ventral conducting chamber, the salivary secretion produced by the glandular cells of pharyngeal mass is poured. 

This contains mucin which lubricates the food and also enzyme protease which digests protein.

The food then passes from the oesophagus and reaches the gizzard which acts as a grinding machine breaking the food. This is done by the contractile movements of the muscular wall and the internal cuticular lining which grinds it still further.

The calciferous glands located in the stomach wall secrete the chalky secretion which neutralizes the humic acid present in the soil.

( Question: Short note on the function of the intestine in Earthworm)

* Function of the intestine in Earthworm 

The intestine is the principal site of digestion. Its wall contains glandular cells which secrete digestive juice containing pepsin, trypsin,  amylase,  lipase and cellulase.

Pepsin: It hydrolyses proteins into proteases and peptones.

Trypsin: hydrolysis of these products into amino acids.

Amylase: hydrolysis starch into maltose.

Lipase: it brings about the hydrolysis of fats into glycerol and fatty acids.

Cellulase: it causes the digestion of cellulose into cellulobiose.

The digestion is thus extracellular in earthworms, like higher animals such as frogs and rabbits.

The intestine also functions for absorbing the digested nutrients by the absorptive cells of the intestinal epithelium, which are then passed to the blood capillaries in the intestinal wall.

Typhosole increases the surface area both for digestion and absorption.

The undigested food and soil are eliminated through the anus in the form of warm casting as small rounded pallets or balls.

Question:

 Difference between the digestive system of earthworms and cockroaches.

Answer:

 There are some major differences in the digestive system of earthworms and cockroaches mostly due to adaptations of the organism and its specific food, feeding habits and nutrition.

Diagram: Comparison of the Digestive System of Earthworm and Cockroach 

Earthworms: The digestive system of earthworms is simpler as they are detritivores ( feeding on decaying organic matter in the soil). It is a straight tube running from the mouth to the anus 

Cockroach: It is more complex in the case of cockroaches as they are omnivores and feed on various organic materials including decaying matter plants and other insects.

Alimentary canal:

Earthworm: The alimentary canal in earthworms is a straight tube extending end to end in the metameric body (the most important characteristics of annelida)

Cockroaches: It is a long and somewhat coiled tube of uneven diameter, divided as foregut, midgut and hindgut, comprising the different organs.

Mouth:

Earthworm: The mouth is a crescentic slit present anteriorly in the peristomium. Suckers and lips are absent.

Cockroaches: They have a mouth cavity or pre-oral chamber, a not-so-well-defined space outside the mouth. The mouth cavity is surrounded by mouthparts consisting upper lip or labrum, mandibles, maxilla, lower lip or labium and hypopharynx.

Pharynx:

Earthworm: Pear-shaped muscular chamber differentiated into a dorsal salivary and a ventral conducting chamber.

Cockroaches: It is short and tubular.

Crop: This functions as storage. This is one of the major differences.

Earthworm: Crop is absent in earthworms as food is readily available hence no need for storage.

Cockroaches: It is a large, thin-walled, pear-shaped sac. The crop serves as a reservoir for storing food.

Gizzard: Both earthworms and cockroaches show the presence of muscular gizzard, but, 

Diagram: T.S. of body through gizzard in Earthworm and Cockroach 

Earthworm: the gizzard is oval with a muscular wall consisting of circular muscle fibres, lined internally by the tough cuticle.

Cockroaches: It is cone-shaped and more complex with armarium and Stomodae-al valve showing six longitudinal folds alternating with six longitudinal grooves with  sharp teeth or denticles

In Earthworm, there is a present

Stomach: Both ends have a sphincter.

 Intestine: extending up to the last 25-26 segments having typhlosole and giving out laterally forwardly directed intestinal caeca.

Rectum: The last part of the post-typhlosolar region about 23-25 segments is the rectum. Internally marked by the presence of longitudinal folds.

Anus: Terminal aperture in the last segment

Cockroaches: The foregut comprises the mouth cavity, mouth, pharynx, oesophagus, crop and gizzard.

After the gizzard starts the midgut which is lined by glandular epithelium serves as the true stomach for digestion and absorption. For this, there are present 7-8 hepatic caeca and 80-90 malpighian tubules.

The next part is differentiated as the hindgut and is divided into the ileum, colon and rectum.

Digestive glands:

Earthworm: Digestive glands are, pharyngeal or salivary gland cells (chromophil cells) on the pharynx which secrete saliva. Also gland cells in the epithelial lining of stomach and intestine.

Cockroaches: In the thorax are present the salivary glands containing zymogenic cells and ductule-containing cells which secrete saliva consisting of enzyme zymase and mucoid substance.

Multiple Choice Questions  (MCQs):

1. In earthworms, the typhlosole is part of the:

a. reproductive tract

b. intestine

c. circulatory system

d. nephridium

Answer: b.intestine

2. Intestinal caeca secretes :

a. proteolytic enzyme (protease)

b. amylase

c. lipase

d. all of the above

Answer: d. all the above 

3. Segment of earthworm in which mouth is found :

a.peristomium

b. prostomium

c. II segment

d. none of these

Answer: a. peristomium

4. In earthworms, unicellular salivary glands are found in :

a. Oesophagus

b. pharynx

c. stomach

d. none of these.

Answer: b.pharynx

5. Alimentary canal of Pheretima internally lined by cuticle in :

a. buccal chamber

b. gizzard

c. both

d. none

Answer: b. gizzard

6. Typhlosole of Pheretima is in :

a. middle and posterior intestine

b. middle intestine and for absorption

c. anterior intestine and for absorption

d. intestine

Answer: b. middle intestine and for absorption.

7. Typhlosole of earthworm starts from the segment :

a. 14

b. 27

c. 20

d. 40

Answer: 27

8. The intestinal caeca in earthworms arise from the segment :

a. 24 and extend forward

b. 26 and extend backward

c. 24 and extend backward

d. 26 and extend forward

Answer: d. 26 and extend forward.

9. Typhlosole of Pheretima serves :

a. to increase the absorptive area of the intestinal epithelium.

b. to slow down the rate of passage of food

c. to secrete digestive juices

d. no purpose 

Answer: a. to increase the absorptive area of the intestinal epithelium.

Next Important topic :

Circulatory System of Earthworm 

Most often asked question:

Life-cycle of Trypanosoma gambiense