Earthworms are hermaphrodites which means that they contain both male and female reproductive organs. After mating, eggs are laid in cocoons where fertilization occurs. After fertilization, the egg begins dividing. By the time an earthworm has reached the four celled-stage, the cellular material is already partitioned into different daughter cells. The fate of these cells has been determined and they would not be capable of developing into a complete organism if allowed to develop independently.
In this article, we will learn about the digestive system and explore the functions of the associated smooth muscle. The region of interest contains the gizzard and the. These structures lie posterior to the oesophagus, which can be easily identified because it is encircled by enlarged blood vessels that function as hearts. The crop-gizzard is also partially obscured by the cream-coloured seminal vesicles of the reproductive system.
The digestive system of the earthworm comprises alimentary canal and glands along with physiology of digestion.
Alimentary canal of earthworm
The alimentary canal is long and extends from the mouth to anus. It consists of the following parts:
1. Mouth: 1st segment
2. Buccal Cavity: 2nd-3rd segment or middle of 3rd segment
3. Pharynx: 3rd-4th segment
4. Oesophagus: 5th -7th segment
5. Gizzard: 8th or 8th-9th segment
6. Stomach: 9th or 10th-14th segment
7. Intestine: 15th up to the last segment except for anus
8. Anus: Last segment
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Mouth: The mouth of an earthworm is a crescentic aperture situated in the 1st segment below the prostomium. The mouth leads into a buccal cavity. Ingestion of food takes place through the mouth.
Buccal Cavity: The buccal cavity is short, wide and a thin-walled tube extending from 2nd up to 3rd or middle of 3rd segment. It has two types of muscle; protractile muscles and retractile muscles. It protrudes out through the mouth with the help of special muscle for holding the food particles during feeding. The buccal cavity leads into the pharynx.
Pharynx: It is a small, swollen, wider, thick-walled pear-shaped chamber that extends till the 4th segment. It is wider than the buccal cavity and differentiated from it using constriction. It has the pharyngeal gland that is located in the dorsal salivary chamber. The pharyngeal gland is made of chromophil cells, which produces saliva containing the proteolytic enzyme; protease and mucin which convert a protein into amino acids and makes the food soft respectively.
Oesophagus: It is a narrow thin-walled tubular structure. The oesophagus extends from the 5th to the7th segment. It has no gland and passes the food particles from the pharynx to gizzard. It leads into the gizzard.
Gizzard: The earthworm gizzard is an oval, thick-walled and highly muscular organ lying in the 8th or 8th-9th segment. It is the hardest part of the alimentary canal because of the presence of the inner lining of the cuticle.
Stomach: The gizzard leads to a short, narrow, thin-walled, vascular tubular structure called the stomach. It extends from the 9th or 10th to 14th segment and wider than the oesophagus. The stomach has a calciferous gland that helps in the neutralisation of food by the calcification process. Stomach leads to the intestine. The glandular cells of the stomach produce proteolytic enzymes which assist in the digestion of protein.
Intestine: The intestine is a long and thin-walled tube which extends from 15th to last segment except anus. The inner lining is ciliated, vascular, folded and glandular. It’s the intestinal lining is folded to form villi. One villus becomes well developed than the other and runs mid-dorsally from 27th to last 25th segment.
Typhlosole Divides the Intestine into 3 regions. They are:
Pre-typhlosolar Region: This region 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, there is a pair of short and conical lateral outgrowth called intestinal caeca that extends upward up to the 23rd segment. Intestinal caeca produces amylase that helps in starch digestion.
Typhlosolar Region: It is the 2nd or middle part of the intestine which extends from the 27th segment to last from the 25th segment. It has both villi and typhlosole. The typhlosole is a vascular and glandular fold that increases the absorptive surface area of the intestine.
Post-typhlosolar Region: It is the last part of the intestine lying in the last 23rd-25th segment in front of the anus and is also called the rectum. It lacks intestinal villi and typhlosole and contains small pellets of mud that are thrown out through the anus to form casting.
Anus: The anus is a circular opening in the last segment called the anal segment. Undigested food materials are released through the anus in the form of worm casting.
Digestive Glands: There are 4 different types of digestive glands associated with the alimentary canal of earthworm.
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Earthworms feed upon all kinds of debris and organic humus such as decaying leaves, microorganism etc. present in soil. They can feed directly on grasses and other vegetation as well.
During feeding the buccal cavity is protruded out with the help of protractile and retractile muscle and the food is drawn into the mouth. The food enters into pharynx through the buccal cavity. The dorsal chamber of pharynx consists of the pharyngeal gland composed of chromophil cell which produces saliva containing mucin and protease. Mucin lubricates the food and turns it to soft food and protease converts protein into amino acids. The food particles then pass through the oesophagus into the gizzard, where it is ground and crushed into the fine state due to the contraction of circular muscles of the gizzard. The ground food material enters into the stomach where the neutralisation of food takes place by a calcification process. Also, the complete digestion of protein by proteolytic enzymes is carried out here. Now the food particles enter into the intestine. In the intestine, intestinal caeca produce amylase which converts starch into glucose.
In 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 exoskeleton of insects.
Lipase: This enzyme converts fats into glycerol and fatty acids.
Digestion takes place mostly in the intestine and the digested food is absorbed by villi. Then it passes into the bloodstream through capillaries. The undigested food and the soil are released out in the form of casting through the anus.
Q1. Write a short note on the Pharynx of an Earthworm.
Ans. Pharynx is the small, wide, thick-walled, pear-shaped chamber. Two chambers are found in pharynx. They are the dorsal chamber and ventral chamber. The dorsal or the salivary chamber which contains the pharyngeal gland composed of chromophil cells. These cells produce saliva containing proteolytic enzymes and mucin. Mucin softens the food soft protease breaks down proteins into amino acids. The ventral chamber is a conducting chamber which conducts digested or undigested food material from the pharynx to the oesophagus. The pharynx is used as a sucking and pumping organ during feeding.
Q2. What are the functions of the Earthworm’s Digestive System?
Ans. Earthworm Digestive System Functions: A summary: Food such as soil enters the earthworm’s mouth where it is taken up and swallowed by the pharynx. Then the soil passes through the oesophagus containing calciferous glands that release calcium carbonate to eliminate the earthworm’s body from extra calcium. After it passes through the oesophagus, the food particles move into the crop where it is stored and then travels into the gizzard. The gizzard uses stones that the earthworm eats to completely grind the food. The food moves into the intestines as gland cells in the intestine release fluids to aid in the digestive process. The intestinal wall has blood vessels where the absorption of the digested food takes place and is transported to the rest of the body.
Q3. Describe the Digestive System of the Earthworm.
Ans. Please refer to the article above