eguruchela

The anatomy of earthworm


The body wall is covered externally by a thin non-cellular cuticle below which is the epidermis, two muscle layers (circular and longitudinal) and an innermost coelomic epithelium.

The epidermis is made up of a single layer of columnar epithelial cells which contain secretory gland cells.

The alimentary canal is a straight tube and runs between first to last segment of the body.

A terminal mouth opens into the buccal cavity (1-3 segments) which leads into muscular pharynx.

A small narrow tube, oesophagus (5-7 segments), continues into a muscular gizzard (8-9 segments).

It helps in grinding the soil particles and decaying leaves, etc. The stomach extends from 9-14 segments.

A terminal mouth opens into the buccal cavity (1-3 segments) which leads into muscular pharynx.

A small narrow tube, oesophagus (5-7 segments), continues into a muscular gizzard (8-9 segments).

It helps in grinding the soil particles and decaying leaves, etc. The stomach extends from 9-14 segments.

morphology of earthworm

The food of the earthworm is decaying leaves and organic matter mixed with soil. Calciferous glands, present in the stomach, neutralise the humic acid present in humus. Intestine starts from the 15th segment onwards and continues till the last segment.

A pair of short and conical intestinal caecae project from the intestine on the 26th segment.

The characteristic feature of the intestine after 26th segment except the last 23rd-25th segments is the presence of internal median fold of dorsal wall called typhlosole.

This increases the effective area of absorption in the intestine.

The alimentary canal opens to the exterior by a small rounded aperture called anus. The ingested organic rich soil passes through the digestive tract where digestive enzymes breakdown complex food into smaller absorbable units. These simpler molecules are absorbed through intestinal membranes and are utilised.

The Pheretima exhibits a closed type of blood vascular system, consisting of blood vessels, capillaries and heart.

Circulatory system

Due to closed circulatory system, blood is confined to the heart and blood vessels.

Contractions keep blood circulating in one direction. Smaller blood vessels supply the gut, nerve cord, and the body wall. Blood glands are present on the 4th, 5th and 6th segments. They produce blood cells and haemoglobin which is dissolved in blood plasma.

Circulatory system of earthworm

Blood cells are phagocytic in nature. Earthworms lack specialised breathing devices. Respiratory exchange occurs through moist body surface into their blood stream.

Nephridial system

The excretory organs occur as segmentally arranged coiled tubules called nephridia and they are of following types:

1. Septal nephridia, present on both the sides of intersegmental septa of segment 15 to the last that open into intestine.

2.Integumentary nephridia, attached to lining of the body wall of segment 3 to the last that open on the body surface.

3.Pharyngeal nephridia, present as three paired tufts in the 4th, 5th and 6th segments.

These different types of nephridia are basically similar in structure. Nephridia regulate the volume and composition of the body fluids.

 of earthworm

A nephridium starts out as a funnel that collects excess fluid from coelomic chamber.

The funnel connects with a tubular part of the nephridium which delivers the wastes through a pore to the surface in the body wall into the digestive tube.

Nervous system

Nervous system is basically represented by ganglia arranged segmentwise on the ventral paired nerve cord. The nerve cord in the anterior region (3rd and 4th segments) bifurcates, laterally encircling the pharynx and joins the cerebral ganglia dorsally to form a nerve ring. The cerebral ganglia alongwith other nerves in the ring integrate sensory input as well as command muscular responses of the body. The nervous system have following components:

Nerve ring
A pair of cerebral ganglia that is present in the third segment lying in the depression between the buccal cavity and pharynx. It elongates laterally and therefore gives rise to a pair of thick connectives that encircle the pharynx and meet ventrally into a pair of ganglia. This makes a complete ring around the pharynx.

Nerve chord
A nerve cord runs backward in the midventral line from ganglia to the posterior of the body. The ventral nerve cord bears ganglion in each segment. The segmental ganglia give off nerves to various parts of the body. The sensory information and the messages are carried from various parts of the body to the nerve cord and vice versa through these nerves.

Sensory system

Sensory system does not have eyes but does possess light and touch sensitive organs (receptor cells) to distinguish the light intensities and to feel the vibrations in the ground. Worms have specialised chemoreceptors (taste receptors) which react to chemical stimuli. These sense organs are located on the anterior part of the worm.

Reproductive system

Earthworm is hermaphrodite (bisexual), means testes and ovaries are present in the same individual.

Male Reproductive System

There are two pairs of testes present in the 10th and 11th segments. The vasa deferentia run up to the 18th segment, they join the prostatic duct. Two pairs of accessory glands are present in the 17th and 19th segments. The common prostate and spermatic duct opens to the exterior by a pair of male genital pores. The male genital pores are present on the ventro-lateral side of the 18th segment. Four pairs of spermathecae are located in 6th to 9th segments. During copulation, spermatozoa are stored in the spermathecae.

Female Reproductive System

One pair of ovaries is attached at the inter-segmental septum of the 12th and 13th segments. Ovarian funnels are present beneath the ovaries. The ovarian funnels continue into oviduct. They join together and open on the ventral side as a single median female genital pore on the 14th segment.

Reproductive system of earthworm

Fertilization and Development

During mating, a mutual exchange of sperms occurs between two worms. Mature sperm and egg cells and nutritive fluid are deposited in cocoons produced by the gland cells of clitellum. Cocoons are deposited in soil. Fertilization and development occur within the cocoons. After about 3 weeks, each cocoon produces two to twenty baby worms. The average number of baby worms from a cocoon is four. Development is direct.

Importance of earthworms

Earthworms are known as ‘friends of farmers’ because they make burrows in the soil and make it porous which helps in respiration and penetration of the developing plant roots.

The process of increasing fertility of soil by the earthworms is called vermicomposting.

They are also used as bait in game fishing.

Phylum Characteristics of Earthworm

Metamerism All annelid bodies are divided serially into segments called metameres. Circular grooves on the outside of their bodies, annuli, separate the metameres. Each segment contains a representative of all major organ systems. Annelids are bilaterally symmetrical.
Body Wall It has an outer circular muscle layer and inner longitudinal muscle layer. By contracting the longitudinal body wall muscles, the body shortens and fattens. When the circular muscles contract, the body lengthens and thins. This allows the worm to burrow, move along the ground or search surrounding water for prey. They have a moist outer cuticle that is secreted by the epithelium which keeps them from desiccating, or drying up.
Chitinous setae Annelids are sometimes called "bristle worms" since they have setae, small hairlike projections from their bodies that aid in burrowing in the ground or swimming through water. They also act to anchor some species in the ground, making it more difficult for predators to pull them from their burrows. Setae are made of chitin. Leeches are the exception as they do not have setae.
Coelom It is the body cavity. In annelids, it is well developed and divided by septa or membranes. The coelom is full of fluid and acts like a hydrostatic skeleton. Since the coelom is divided by septa the worm can move restricted areas of its body separately.
Closed Circulatory System The circulatory system is closed means that the blood is enclosed in muscular blood vessels. They have aortic arches, which act as hearts to pump the blood.
Complete Digestive System It is complete means it includes a separate mouth and anus, connected by a foregut, midgut and hindgut.
Respiration Respiration in annelids takes place directly through the skin, through gills or through parapodia which are leg-like structures found in some annelids. Earthworms take in oxygen and give off carbon dioxide directly through their skin.
Excretory System In each metamere, there are a pair of nephridia, which are a type of primitive kidney. Coelomic fluid is filtered through the nephridia via the nephrostome. The nephridia resorb nutrients in the nephridial duct and the waste is excreted through the nephridiopore located by the ventral setae on the earthworm.
Nervous System There is a double ventral nerve cord with a pair of ganglia and lateral nerves at each metamere. The annelid brain is rudimentary, composed of a pair of dorsal cerebral ganglia.