Pheritima and Lumbricus are the two common species of earthworms belonging to the phylum Annelida found in India. They are found in the soil and help make it fertile by feeding on dead organic matter.

Earthworms are hermaphrodites with long, cylindrical, and segmented bodies. Their digestive system consists of an alimentary canal running along the length of the body from mouth to anus, as well as glands.

Table of Contents

Characteristics of an Earthworm

[Structure of the Alimentary Canal](#Structure of the Alimentary Canal)

[Physiology of Digestion](#Physiology of Digestion)

[Frequently Asked Questions](#Frequently Asked Questions)

Characteristics of an Earthworm

• Can be found in moist soil
• Have a segmented body
• Have a muscular, tube-like body
• Have a simple nervous system
• Have a complete digestive system
• Have a closed circulatory system
• Have no eyes or ears
• Have a small brain
• Breathe through their skin

The main characteristic features of an earthworm are:

• Long cylindrical body
• Segmented body
• Lack of eyes and ears
• Presence of setae
• Possess a digestive system
• Capable of reproducing both sexually and asexually

Long, cylindrical, and segmented body

It is a hermaphrodite, possessing both male and female sexual organs.

It has a hydrostatic skeleton

It has a central and peripheral nervous system

The setae present in the epidermal pits help with locomotion.

The epidermis is composed of columnar epithelial cells and contains secretory gland cells.

The digestive system runs throughout the body

It breathes through the skin

It has a closed type of blood vascular system, comprised of capillaries, vessels, and a heart.

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Digestive System of Earthworms

The digestive system of an earthworm consists of an alimentary canal that runs along the length of the body from the mouth to the anus, as well as glands.

The Structure of the Alimentary Canal

The alimentary canal is long and straight and runs between the first and last segment of the body.

| Mouth | 1 |

| Buccal Cavity | 2-3 |

| Pharynx | 3-4 |

| Oesophagus | 5-7 cm |

| Gizzard | 8-9 |

| Stomach | 9-14 |

| Intestine | 15-Last |

| Anus | Last |

Mouth: The first segment of an earthworm is known as the peristomium and is covered by the prostomium, which serves to create a gap in the soil. This prostomium is sensory in nature. The mouth of an earthworm is a crescent-shaped opening that leads to a buccal cavity. Food is taken in through the mouth.

Buccal Cavity: This cavity extends from the second to the third segment and is a thin-walled chamber. It is able to protrude out or retract in with the assistance of the contraction of muscles attached to the body wall. Additionally, the buccal cavity helps to hold food during ingestion and opens into a muscular chamber known as the pharynx.

Pharynx: It is a thick-walled, pear-shaped chamber that extends to the fourth segment. It is distinguished from the buccal chamber by a dorsal groove, which houses the earthworm’s brain. The dorsal part of the pharynx has a pharyngeal bulb composed of muscle fibers, connective tissue, blood vessels, and salivary glands. The salivary glands are unicellular and known as chromophil cells, which secrete saliva containing proteolytic enzymes, such as protease, which break down proteins into amino acids. Additionally, mucin is another enzyme found in saliva that helps to soften food. The pharyngeal wall is connected to the body wall through muscular strands which can contract or dilate the pharyngeal lumen, acting as a suction pump.

Oesophagus: The pharynx continues into the oesophagus, which is a small, narrow, thin tube-like structure present on 5-7 segments. It passes food to the gizzard and has no glands.

Gizzard: It is present on 8-9 segments and is a highly muscular, thick-walled organ. Its inner lining is composed of a cuticle, making it the toughest part of the alimentary canal. It aids in the grinding of soil particles and decaying leaves.

Stomach: The stomach extends from 9-14 segments, and is a highly vascular and tubular structure. Three pairs of Calciferous glands, located in the segments 10-12, produce carbonate of lime which helps to neutralise humic acid present in the humus. Glandular cells of the stomach secrete proteolytic enzymes which aid in the digestion of proteins. The stomach ultimately leads to the intestine.

Intestine: It is a long, thin-walled tube that begins at the 15th segment and ends at the anus (the last segment). The inner lining of the intestine is ciliated, vascular, and glandular and is folded to form villi, which increase the effective absorption area of the intestine. The intestine is divided into three parts:

Pre-typhlosolar Region: Present from 15-26 segments, the pre-typhlosolar region contains villi and a short conical projection on the 26th segment known as intestinal caeca. Intestinal caeca extend upwards till the 23rd segment and secrete the amylase enzyme, which helps in the digestion of carbohydrates.

Typhlosolar Region: The Typhlosole is a large internal median fold of the dorsal wall of the intestine, forming a longitudinal ridge which appears after the 26th segment, with the exception of the last 23rd-25th segments. This Typhlosole increases the area of absorption in the intestine.

Post-typhlosolar Region: This region is the last 25 segments of the intestine and is also known as the rectum. It does not contain any intestinal villi or typhlosole. This region stores faecal pallets and leads to the anus.

Anus: It is a small, round aperture located in the last segment of the alimentary canal of an earthworm. The anus functions as an opening through which undigested food is excreted out in the form of worm castings.

Physiology of Digestion

Earthworms feed on dead and decaying organic matter present in the soil, which they gather at night. Their digestion is an extracellular process.

| The mouth and buccal cavity play a role in ingestion. Additionally, there are many chemoreceptors located near the mouth region which assist in recognizing chemicals and food. |

| The contraction of muscles that extend from the pharynx to the body wall enlarges the pharyngeal cavity, drawing food particles into the buccal cavity. |

The food is acted upon by proteolytic enzymes present in the pharynx, which hydrolyze proteins. Mucin also helps to lubricate the food.

| The food enters the gizzard after passing through the oesophagus. The gizzard acts like a grinder, breaking the food down into small particles through its squeezing and rolling action. |

| The food that is ingested enters the stomach, where calciferous glands aid in neutralizing the acidity of the food. |

| The food then moves to the intestine, where amylase present in the intestinal caeca breaks down starch into glucose. |

The intestine is where complete digestion of food takes place. Various enzymes present in the intestine act on food, breaking it down into smaller molecules that can be absorbed. Typhlosole increases the absorption surface area of the intestine, allowing for greater absorption of digested food.

The undigested food passes to the rectum where water is absorbed and the undigested food is then excreted out through the anus in the form of castings.

The Intestinal Enzymes Are:

Protease: Protein to Amino Acids

Amylase: Starch -> Glucose

Lipase: Converts Fats into Fatty Acids and Glycerol

Cellulase: Conversion of Cellulose to Glucose

Chitinase: hydrolyzes glycosidic bonds in chitin

Frequently Asked Questions

Typhlosole

Typhlosole is an anatomical feature of some animals, consisting of a thickening of the epidermis, which forms a vertical ridge down the middle of the back. It is believed to improve the animal’s ability to sense its environment, as the ridge increases the sensitivity of the skin. Typhlosoles are found in a variety of animals, including some reptiles, amphibians, and fish.

Typhlosole is a large internal median fold of the dorsal wall of the intestine, forming a longitudinal ridge that is present after the 26th segment, except for the last 23rd-25th segments. This fold increases the area of absorption in the intestine.

What Enzymes are Present in an Earthworm?

The digestion process in earthworms is an extracellular process. Food is broken down outside the cell with the help of enzymes such as protease, amylase, lipase, cellulase and chitinase.