Detailed Study of Earthworm

Habit and Habitat

• Earthworms are terrestrial and burrowing organisms with a cosmopolitan or worldwide distribution, occurring mostly in tropical and subtropical regions with moist and damp soil.
• They are detritivorous feeders and feed on dead and decaying organic vegetation, kitchen wastes, plant leaves and soil.
• They are commonly found in damp and moist places rich in organic vegetation.
• Earthworms are nocturnal. The source states that they remain inside burrows during sunny days and come out at night and during cloudy or rainy weather.
• Breeding occurs during the rainy season. Earthworm is hermaphrodite or bisexual but shows cross-fertilization due to the relative positions of male and female genital apertures and its protandrous condition, in which sperm mature earlier than eggs.

External Features

1. Shape and Size

The source describes an earthworm as bilaterally symmetrical, cylindrical and elongated. The anterior end is tapering, while the posterior end is more or less blunt. A mature worm measures about 150 mm in length and about 3–5 mm in width.

The dorsal surface is darker and shows a dark mid-dorsal line due to the thick dorsal blood vessel lying just beneath the semi-transparent skin. The ventral surface is somewhat yellowish to pale and bears genital apertures and papillae in the anterior region.

2. Colouration

The body is slimy to touch and appears brown due to the presence of porphyrin pigment in the body wall. The notes state that the pigment protects the body from the injurious effect of bright light. The dorsal surface is darker than the ventral surface.

3. Segmentation

The body consists of about 100–200 small ring-like segments called metameres or somites. It shows true segmentation because the external segments correspond with internal segments. This is called metameric segmentation. The segmentation is formed by intersegmental septa.

4. Setae

Each segment except the first, last and clitellar segments bears an equatorial annular row of minute, S-shaped, yellowish setae. The source states that about 80–120 setae per segment may be present. They project backward from the body surface and serve as locomotory organs.

Locomotion involves setae, the musculature of the body wall and a hydraulic skeleton. The notes state that pressure in the coelomic fluid makes the cuticle hard, forming a hydraulic skeleton, which also helps in burrowing.

5. Clitellum

About 2 cm behind the front end, a mature earthworm bears a thick collar-like or girdle-like glandular thickening of the body wall called the clitellum. It occurs in the 14th, 15th and 16th segments.

During breeding, glandular cells of the clitellum secrete mucus, albumen and material for cocoon formation. Because of the clitellum, the body is divided into pre-clitellar, clitellar and post-clitellar regions. The pre-clitellar region has 13 segments, the clitellar region comprises segments 14–16, and the post-clitellar region begins from the 17th segment.

6. Body Wall

The body wall consists of cuticle, epidermis and musculature. The epidermis is single-layered but multicellular. The musculature consists of outer circular and inner longitudinal muscles.

External Apertures

Digestive System in Earthworm

The digestive system is concerned with ingestion, digestion and absorption of food and the egestion of undigested waste. The source identifies two essential components: the alimentary canal or digestive tract and associated digestive glands.

A. Alimentary Canal

The alimentary canal is a long, straight, tubular structure of varying diameter, extending from the mouth to the anus.

1. Mouth and Buccal Chamber

The mouth is a small crescentic aperture in the first segment below the peristomium. It leads into a small pear-shaped, thin-walled buccal chamber extending up to the middle of the third segment. The buccal chamber can protrude out through the mouth during feeding.

2. Pharynx

The pharynx is a short, thick, pear-shaped structure extending from the buccal chamber to the oesophagus. A horizontal groove divides the pharyngeal lumen into two chambers. The dorsal salivary chamber secretes mucin and proteolytic enzymes. Mucin lubricates food, while the source states that proteolytic enzymes digest proteins.

3. Oesophagus

The oesophagus is a short, narrow and muscular structure extending from the 5th to the 7th segment, between the pharynx and gizzard. It passes food from the pharynx to the gizzard by peristalsis.

4. Gizzard

The gizzard is a prominent, oval, muscular structure situated mainly in the 8th segment, between the oesophagus and stomach. It contains thick circular muscles that pulverize food from large pieces into smaller pieces and acts as a grinding organ. The source describes its inner lining as having a tough cuticle that acts like teeth during grinding.

5. Stomach

The stomach is a narrow, muscular, glandular and highly vascular tubular structure extending approximately from the 9th to 14th segment. Calciferous glands in its inner lining secrete calcium carbonate. The source states that calcium ions help neutralize humic acid in soil, while carbonate is eliminated along with undigested waste. The stomach wall also secretes proteolytic enzymes.

6. Intestine

The intestine is a long, thin-walled, wide tubular structure beginning behind the 14th segment. Its inner lining is ciliated, vascular and highly glandular and contains folded structures called villi.

Feeding Mechanism and Physiology of Digestion

Feeding Mechanism

The source describes earthworm as omnivorous and detritivorous. It feeds on small protozoans, nematodes, small insects, vegetables, plant leaves and other organic wastes together with soil.

During feeding, the earthworm presses its head against the soil surface. The pharynx acts as a suction pump. During contraction, the buccal cavity protrudes through the mouth; during relaxation, the buccal cavity and food enter the alimentary canal.

Physiology of Digestion

1. No digestion takes place in the buccal cavity.
2. Saliva from pharyngeal glands contains mucin and proteolytic enzymes. Mucin lubricates food and proteolytic enzymes begin protein digestion.
3. Food moves from pharynx to gizzard through the oesophagus by peristaltic movement.
4. In the gizzard, physical digestion occurs as the food is ground into smaller pieces.
5. In the stomach, calciferous gland secretion neutralizes humic acid and proteolytic enzymes continue protein digestion.
6. In the intestine, different digestive enzymes act on different food components.

Most digested food is absorbed through the inner lining of the pre-typhlosolar and typhlosolar regions. Some water is absorbed in the post-typhlosolar region, while the remaining undigested material is egested through the anus.

Excretory or Nephridial System in Earthworm

Excretion is performed by long, thin and coiled tubules called nephridia. The source states that nephridia occur in almost all body segments except the first three.

1. Septal Nephridia

Septal nephridia are the largest and well-developed nephridia. They occur in segments behind approximately the 15th segment, with many nephridia present on each side of a septum. They collect excretory wastes from the coelomic cavity and pass them into the intestine through supra-intestinal excretory ducts, so the source calls them enteronephric nephridia.

A typical septal nephridium is described as having three main parts:

2. Pharyngeal Nephridia

Three pairs of pharyngeal nephridia are situated in the 4th, 5th and 6th segments on either side of the alimentary canal. Ducts from the 4th and 5th segments open into the pharynx, while the source states that a duct from the 6th segment opens into the buccal cavity. These are enteronephric because they open into the gut.

3. Integumentary Nephridia

These are found in most body segments except the first few. The source states that approximately 200–250 integumentary nephridia occur in many segments and about 2000–2500 occur in the clitellar region. Because of this abundance, the clitellum is called the forest of nephridia. They are exonephric because they open directly outside the body.

Nervous System in Earthworm

The source describes the nervous system as well developed and divides it into three parts:

1. Central Nervous System

The central nervous system is the major controlling part. The source describes a nerve ring around the pharyngeal region and a ventral nerve cord.

A pair of thick-walled cerebral ganglia or the brain lies dorsally between the buccal cavity and pharynx. Cerebral ganglia connect with sub-pharyngeal ganglia through circumpharyngeal or peripharyngeal connectives on either side of the pharynx. Behind this, a double-layered tubular ventral nerve cord extends posteriorly and bears segmental ganglia.

2. Peripheral Nervous System

This system includes nerves arising from the central nervous system. The notes state that several peripheral nerves arise from the cerebral ganglia and other ganglia. Peristomial nerves carry impulses toward the peristomium and buccal region. Nerves from the sub-pharyngeal ganglia supply the anterior segments, while three pairs of peripheral nerves arise from each segmental ganglion to supply structures in the respective segments.

Peripheral nerves are mixed, containing afferent or sensory neurons that carry impulses from receptors toward the central nervous system and efferent or motor neurons that carry impulses from the central nervous system toward effectors.

3. Sympathetic or Autonomic Nervous System

This system includes extensive nerve plexuses in the body-wall musculature and the gut wall. The source states that these later connect with the circumpharyngeal connectives.

Reproductive System in Earthworm

Male Reproductive Organs

1. Two Pairs of Testes

Two pairs of testes are present in the 10th and 11th segments and are enclosed in testis sacs. The source describes each testis as having a narrow base attached to the anterior inner wall of its testis sac and a broader end with numerous finger-like processes. Sperm are formed by spermatogenesis and immature sperm pass toward seminal vesicles for maturation.

2. Two Pairs of Testis Sacs

Two pairs of whitish, double-layered sac-like testis sacs occur in the 10th and 11th segments. The testis sac of the 11th segment is generally larger because of an additional pair of seminal vesicles. They serve as protective coverings around testes, spermiducal funnels and seminal vesicles.

3. Two Pairs of Spermiducal Funnels

Two pairs of ciliated, funnel-like spermiducal funnels occur in the 10th and 11th segments. Their ciliated margins help capture sperm and pass them into the vasa deferentia toward the prostate region.

4. Seminal Vesicles

Two pairs of oval or rounded seminal vesicles occur around the 11th and 12th segments. The source states that they help in maturation of sperm.

5. Vasa Deferentia

Two pairs of muscular, tubular and ciliated structures arise from the spermiducal funnels and extend toward the prostate gland. Their cilia help pass sperm toward the prostate region.

6. Prostate Glands

A pair of large, white, solid and irregularly shaped glandular masses lies on either side of the gut, extending across several anterior segments. Each gives rise to a curved prostate duct. The source states that the ducts join the two vasa deferentia of the respective side and form a common prostatic-spermatic duct.

7. Accessory Glands

Two pairs of rounded accessory glands occur on the ventral body wall around the 17th and 19th segments. They open outside by small ductules on genital papillae and assist during copulation.

Female Reproductive Organs

1. Ovaries

A pair of small, irregular and lobulated ovaries is situated on the posterior wall of the septum between the 12th and 13th segments, one on either side of the ventral nerve cord. Eggs are arranged in the lobules in order of development.

2. Oviducal Funnels

A pair of funnel-shaped or saucer-shaped ciliated oviducal funnels is arranged in the intersegmental septum between the 13th and 14th segments, one on either side of the ventral nerve cord. The cilia help capture eggs.

3. Oviducts

A pair of tubular ciliated oviducts arises from the oviducal funnels. The two oviducts fuse and open through the female genital aperture through which eggs are released after copulation.

4. Spermathecae

Four pairs of spermathecae occur around the 6th, 7th, 8th and 9th segments. Each spermatheca has a shorter diverticulum that helps store sperm after copulation and a wider pear-shaped ampulla that provides nourishment to sperm.

Spermathecae open to the exterior through spermathecal pores in intersegmental regions. These pores allow sperm to enter or leave the spermathecae.

Copulation

The source states that copulation occurs during the rainy season, at night or early morning, and lasts about an hour. Two worms come close and apply their ventral surfaces together while keeping their heads in opposite directions. The male genital apertures of one worm align with the spermathecal pores of the other, permitting exchange of sperm.

Cocoon Formation

Cocoon formation starts after copulation. The source states that many cocoons may be formed after each copulation due to periodic release of sperm from spermathecal pores.

Clitellar segments contain three types of glands:

The girdle moves forward toward the head because of backward wriggling. As it crosses the female genital aperture, eggs enter the cocoon. As it passes over spermathecal pores, stored sperm enter the cocoon. Finally the girdle is thrown off from the anterior end, and its elasticity closes both ends to form the cocoon.

Economic Importance of Earthworm

Beneficial Aspects

Harmful Aspects