The earthworm is studied under Phylum Annelida of the animal kingdom. They are known as farmer’s friends because they play a significant role in agriculture. The faecal deposit of earthworms is known as worm casting, which increases the fertility of the soil and thus the yield. It also increases soil aeration, infiltration, structure and nutrient cycling and water movement in the soil. They are one of the major decomposers of organic matter. They get their nutrition from microorganisms living on organic matter and soil material. There are about 500 species, of which 13 are found in India. Some common species include Pheretima Posthumaa and Lumbricus. They are terrestrial animals found in the uppermost moist layer of soil. This article briefly explains the morphology and anatomy of their functions.

Morphology of Earthworms

Earthworms have long cylindrical reddish-brown bodies divided into 100-120 similar short segments known as metameres. 

• The dorsal surface of the body has a dark median, mid-dorsal line along the longitudinal axis of the body, and the presence of genital openings distinguishes the ventral surface. 
• The anterior end of the first body segment consists of the mouth and the prostomium, covering the mouth, which is used as a muscular probe in burrowing and sensory functions as they do not have sense organs. 
• Mature worms have clitellum at the 14th to 16th segment, secreting cocoons.
• Four pairs of spermathecal apertures in males are present at the 5th to 9th segment and 18th segment.
• In females, a single genital pore is present in the mid-ventral line of the 14th segment. 
• Several minute pores are present on the body surface called nephridiopores, which are excretory organs.
• There are rows of S-shaped setae that help in locomotion in each segment except the first, last and clitellum.

Anatomy

An earthworm’s body comprises four layers externally by a thin, non-cellular layer called a cuticle. Below it, the epidermis is present, consisting of a single layer of columnar cells containing gland cells and sensory cells. Two muscular layers are present: a thin, outer layer of circular muscle fibres, a thick inner layer of longitudinal muscle fibre and an innermost coelomic epithelium. A different system of the body that constitutes the anatomy is explained;

1.   Digestive system

Earthworms feed on dead organic matter and decaying leaves in the soil. The earthworm’s digestive system comprises the alimentary canal and various associated digestive glands. The alimentary canal is a complete and straight tube that runs between the body’s first to the last segment. 

• The mouth is the terminal aperture that opens into the buccal cavity that extends from the 1st to the 3rd segment, followed by the pharynx. 
• The pharynx leads to a small narrow tube, the oesophagus, extending from the 5th to the 7th segment.
• In segments 8th and 9th, a muscular gizzard is present that helps grind the soil particles and decaying leaves, etc., with the help of its thick muscles and cuticles. 
• The gizzard is followed by a short narrow tubular stomach extending from the 9th to the 14th segments with calciferous glands that neutralise the humic acid found in the humus.
• The intestine extends from the 15th segment to the last segment. In the 26th segment, a pair of short and conical intestinal caeca is present in the next 3 to 4 segments.
• The 26th to the 95th segments have the presence of an internal median fold of the dorsal wall called Typhlosole. This increases the effective area of digestion and absorption in the intestines.
• At last, the alimentary canal opens to the exterior by a small rounded aperture called the anus.

Once the pumping action of its pharynx ingests the food, the organic-rich food passes through different parts of the digestive tract. In gizzards, food is ground thoroughly in the stomach, and the calciferous glands neutralise the humic acid present in it. The intestinal wall consists of glandular cells that secrete digestive juice containing digestive enzymes, which break down complex food into small absorbable units that are absorbed through absorptive cells of the intestinal epithelium and utilised in the body. The undigested food matter and the soil are passed out through the anus in little heaps or pellets called worm casting, which increases soil fertility.

2. Circulatory system

Earthworms exhibit a closed type of blood vascular system; i.e., blood is confined to the heart and blood vessels.  Blood is composed of fluid plasma, colourless blood corpuscles and haemoglobin, which gives a red colour to the blood and helps in O2 transport for respiration.  Blood cells are produced by the blood glands present at the 4th, 5th and 6th segments. 

• Four pairs of tubular hearts are enlarged and thick-walled pulsatile with valves.
• The anterior two pairs are lateral hearts lying in the 7th and 9th segments connecting dorsal blood vessels with ventral vessels. 
• The posterior two pairs are called latero-oesophageal hearts in the 12th and 13th segments. They are thick, muscular and rhythmically contractile vertical vessels.
• A pair of thin-walled, non-pulsating loop-like vessels without valves known as anterior loops are present in the 10th and 11th segments.

3. Respiratory system

Pheretima lacks special respiratory organs; therefore, the gaseous exchange occurs through the general body surface into their bloodstream. The body surface is kept moist by the secretion of epidermal cells and coelomic fluid. O2 diffuses through the skin, combines with haemoglobin, and circulates in the body tissues, where it helps in the oxidation of food to release CO2, water and energy. Again CO2 diffuses out to the environment through the skin; hence it is known as cutaneous respiration.

4. Excretory system

The excretory organs of earthworms are coiled tubules, segmentally arranged called nephridia. These are of three types-

1. Septal nephridia- present on both the sides of intersegmental septa of segment 15 to the last that open into the intestine
2. Integumentary nephridia- attached to the lining of the body wall of segment 3 to the last that opens on the body surface.
3. Pharyngeal nephridia- three paired tufts in the 4th, 5th and 6th segments.

A nephridium starts like a funnel that connects with a tubular part of the nephridium and terminates at pores called nephridiopores. Nephridia regulate the volume and composition of body fluids, as they are abundantly supplied with the blood vessels. They bear gland cells that extract excess water and nitrogenous waste materials to the outer body surface through pores. Pharyngeal and septal nephridia discharge them into the gut lumen from where they are eliminated with faeces.  

5. Nervous system and its function

The nervous system is made up of ganglia that are placed segment by segment on the ventral paired nerve cord. The anterior nerve cord (3rd and 4th segments) bifurcates and forms a nerve ring surrounding the pharynx and connecting to the brain ganglia dorsally. Earthworms have special chemoreceptors and touch-sensitive organs since they lack sensory organs.

6. Reproduction in earthworms

The earthworm is a hermaphrodite, which means that it has male and female reproductive organs. They have two pairs of testes in the 10th and 11th segments, vasa deferentia run-up to the 18th segment, and two accessory glands in the 17th and 19th segments. Four pairs of spermathecae are located in the 6th to 9th segments. 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. Mating is juxtaposing, i.e., worms apply each other by their ventral surfaces with heads in opposite directions. A mutual exchange of egg cells and mature sperm and nutritive fluid (prostatic fluid), etc., gets deposited in cocoons produced by the clitellum gland cells. Fertilisation and embryo development occur within the cocoons. Each cocoon produces two to twenty baby worms after three weeks, and the direct development of worms occurs without larva formation.

Conclusion

Earthworms are reddish-brown in colour due to the presence of porphyrin and can replace lost segments. They have photoreceptors on the dorsal surface of the body and in the anterior part, which helps them judge the intensity and duration of light. Their morphological and anatomical organisation makes them creatures of great economic importance. They feed on dead organic matter, decaying leaves, and soil, which makes them fertile, and their burrowing habit makes the soil loose and porous. Worm castings of the earthworm are used as manure, and the process of increasing the fertility of the soil by earthworms is called vermicomposting. They reduce the acidity and alkalinity of the soil and thus create optimum conditions for plant growth with their digestive and excretory functions.