• The earthworm is a reddish-brown terrestrial invertebrate that inhabits the upper layer of moist soil.

• During day time, they live in burrows made by boring and swallowing the soil.

• In the gardens, they can be traced by their fecal deposits known as worm castings.

  • The typical Indian earthworms are Pheretima and Lumbricus.

Morphology:

• Earthworms have long cylindrical bodies.

• The body is divided into more than a hundred short segments which are similar (metameres about 100-120 in number).

• The dorsal surface of the body is marked by a dark median mid-dorsal line (dorsal blood vessel) along the longitudinal axis of the body.

• The ventral surface is distinguished by genital openings (pores).

• The anterior end consists of the mouth and the prostomium, a lobe that serves as a covering for the mouth and a wedge to force open cracks in the soil into which the earthworm may crawl.

  • The prostomium is sensory in function.

• The first body segment is called the peristomium (buccal segment) which contains the mouth.

• In a mature worm, segments 14-16 are covered by a prominent dark band of glandular tissue called clitellum.

  • Thus the body is divisible into three prominent regions – preclitellar, clitellar and postclitellar segments.

• Four pairs of spermathecal apertures are situated on the ventrolateral sides of the intersegmental grooves, i.e., 5th -9th segments.

• A single female genital pore is present in the mid-ventral line of the 14th segment.

• A pair of male genital pores are present on the ventrolateral sides of the 18th segment.

• Numerous minute pores called nephridiopores open on the surface of the body.

• In each body segment, except the first, last, and clitellum, there are rows of S-shaped setae, embedded in the epidermal pits in the middle of each segment.

• Setae can be extended or retracted.

  • Their principal role is in locomotion.

Anatomy:

• The body wall of the earthworm 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 that runs between the first to the last segment of the body.

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

• A small narrow tube, the esophagus (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.

• The food of the earthworm is decaying leaves and organic matter mixed with soil.

• Calciferous glands, present in the stomach, neutralize the humic acid present in the humus.

• The intestine starts from the 15th segment onwards and continues till the last segment.

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

• The characteristic feature of the intestine after the 26th segment except the last 23rd-25th segments is the presence of an internal median fold of the 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 the anus.

• The ingested organic-rich soil passes through the digestive tract where digestive enzymes break down complex food into smaller absorbable units.

  • These simpler molecules are absorbed through intestinal membranes and are utilized.

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

• Due to a 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 body wall.

• Blood glands are present on the 4th, 5th, and 6th segments.

  • They produce blood cells and hemoglobin which is dissolved in blood plasma.

  • Blood cells are phagocytic in nature.

• Earthworms lack specialized breathing devices.

  • Respiratory exchange occurs through a moist body surface into the bloodstream.

• The excretory organs occur as segmentally arranged coiled tubules called nephridia.

  They are of three types:

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

  • integumentary nephridia, attached to the lining of the body wall of segment 3 to the last that open on the body surface

  • 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 body fluids.

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

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

• The 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 along with other nerves in the ring integrate sensory input as well as command muscular responses of the body.

• The 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 specialized chemoreceptors (taste receptors) that react to chemical stimuli.

  • These sense organs are located on the anterior part of the worm.

• The earthworm is hermaphrodite (bisexual), i.e., testes and ovaries are present in the same individual.

• There are two pairs of testes present in the 10th and 11th segments.

  • Their vasa deferentia run up to the 18th segment where they join the prostatic duct.

  • Two pairs of accessory glands are present one pair each in the 17th and 19th segments.

  • The common prostate and spermatic duct (vasa deferentia) are open to the exterior by a pair of male genital pores on the ventrolateral side of the 18th segment.

  • Four pairs of spermathecae are located in the 6th-9th segments (one pair in each segment).

    • They receive and store spermatozoa during copulation.

• One pair of ovaries is attached at the inter-segmental septum of the 12th and 13th segments.

  • Ovarian funnels are present beneath the ovaries which continue into the oviduct, join together and open on the ventral side as a single median female genital pore on the 14th segment.

  • A mutual exchange of sperm occurs between two worms during mating.

• One worm has to find another worm and they mate juxtaposing opposite gonadal openings and exchanging packets of sperms called spermatophores.

• Mature sperm and egg cells and nutritive fluid are deposited in cocoons produced by the gland cells of the clitellum.

• Fertilization and development occur within the cocoons which are deposited in the soil.

• The ova (eggs) are fertilized by the sperm cells within the cocoon which then slip off the worm and are deposited in or on the soil.

• The cocoon holds the worm embryos.

• After about 3 weeks, each cocoon produces two to twenty baby worms with an average of four.

• The development of earthworms is direct, i.e., there is no larva formed.

• 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 the fertility of the soil by the earthworms is called vermicomposting.

• They are also used as bait in game fishing