An earthworm is an invertebrate belonging to the phylum Annelida. They are found in the soil and make it fertile. They feed on dead organic matter. Pheritima and Lumbricus are the common earthworms found in India. The main characteristics of an earthworm are:
- Long, cylindrical and segmented body
- It is a hermaphrodite, having both male and female sex organs
- It has a hydrostatic skeleton
- It has a central and peripheral nervous system
- The setae present in the epidermal pits help in locomotion
- The epidermis is made up of columnar epithelial cells. It has secretory gland cells
- The digestive system runs along the body
- It respires through the skin
- It has a closed type of blood vascular system, which is made up of capillaries, vessels and a heart
Digestive system of Earthworms
The digestive system of earthworm comprises an alimentary canal that runs along the length of the body from mouth to anus and glands.
Structure of the Alimentary Canal
The alimentary canal is long and straight and runs between the first and last segment of the body.
|Parts of the alimentary canal||No. of the segment in the body|
- Mouth: Mouth is the first segment called peristomium and is covered by prostomium, which acts as a wedge to force an open crack in the soil. The prostomium is sensory in nature. The mouth is a crescentic aperture which opens into the buccal cavity. Food ingestion takes place through mouth.
- Buccal Cavity: It extends from 2nd to 3rd segment. It is a thin-walled chamber. It can protrude out or retract in with the help of contraction of muscles attached to the body wall. It helps in holding the food during ingestion. Buccal cavity opens into a muscular chamber called the pharynx.
- Pharynx: It’s a thick-walled, pear-shaped chamber, which extends till 4th segment. It is differentiated from the buccal chamber by a dorsal groove where the brain of an earthworm is present. The dorsal part of the pharynx has a pharyngeal bulb, which is made up of muscle fibre, connective tissues, blood vessels and salivary glands. Salivary glands are unicellular and known as chromophil. Chromophil cells secrete saliva which contains proteolytic enzymes protease which converts protein to amino acids. Mucin is the other enzyme present, which makes the food soft. The pharyngeal wall is connected to the body wall through muscular strands which contract or dilate the pharyngeal lumen. It acts as a suction pump.
- Oesophagus: Pharynx continues into the oesophagus. It is a small narrow thin tube-like structure present on 5-7 segment. It passes food to the gizzard. It has no glands.
- Gizzard: It is present on 8-9 segment. It is a highly muscular thick-walled organ. It has an inner lining of the cuticle which makes it the hardest part of the alimentary canal. It helps in the grinding of soil particles and decaying leaves.
- Stomach: The stomach extends from 9-14 segments. It is a highly vascular and tubular structure. Three pairs of Calciferous glands are present in the segment 10-12, which produces carbonate of lime and helps in neutralising humic acid present in the humus. Glandular cells of the stomach secrete proteolytic enzymes which help in digestion of proteins. Stomach leads to the intestine.
- Intestine: It is a long thin-walled tube present from the 15th segment till anus i.e. the last segment. The inner lining of the intestine is ciliated, vascular and glandular. The inner lining is folded to form villi. These villi increase the effective area of absorption in the intestine. The intestine is divided into 3 parts:
- Pre-typhlosolar region: present from 15-26 segment. It contains villi. There is a short conical projection on the 26th segment, which is known as intestinal caeca. Intestinal caeca extend upwards till 23rd segment and secrete amylase enzyme. Amylase helps in the digestion of carbohydrates.
- Typhlosolar region: Typhlosole is the large internal median fold of the dorsal wall of the intestine forming a longitudinal ridge, which is present after the 26th segment, except the last 23rd-25th segments. Typhlosole increase the area of absorption in the intestine
- Post-typhlosolar region: It’s the last 25 segments of intestine and also known as rectum. Intestinal villi and typhlosole are absent in this region. It stores faecal pallets and leads to the anus.
- Anus: It is a small round aperture present in the last segment. The alimentary canal of an earthworm opens to the exterior through the anus. Undigested food is excreted out through the anal opening in the form of worm casting.
Physiology of Digestion
Earthworm feeds on dead and decaying organic matter present in the soil. This material is gathered at night. The digestion of earthworm is an extracellular process.
|The mouth and the buccal cavity takes part in the ingestion. There are many chemoreceptors present near the mouth region which help in identifying chemicals and food they eat.|
|Food particles are drawn into the buccal cavity when the pharyngeal cavity is enlarged by the contraction of muscles that extend from pharynx to the body wall.|
|The food is acted on by proteolytic enzymes present in pharynx which hydrolyse proteins and mucin helps in the lubrication of food.|
|The food comes to gizzard passing through oesophagus. The gizzard works like a grinder. The food is broken into minute particles by squeezing and rolling action.|
|The ground food enters the stomach where calciferous glands help in neutralizing the acid of the food.|
|Then the food moves to the intestine, here amylase present in the intestinal caeca convert starch into glucose.|
|Complete digestion of food takes place in the intestine. Various enzymes present in the intestine act on food and digested food gets absorbed. Typhlosole increases the absorption surface of the intestine.|
|The undigested food passes to the rectum where water gets absorbed and the undigested food is egested out through the anus in the form of castings.|
The enzyme present in the intestine are:
- Protease: Protein to amino acids
- Amylase: Starch to glucose
- Lipase: Fats to fatty acid and glycerol
- Cellulase: Cellulose to glucose
- Chitinase: hydrolyse glycosidic bonds in chitin