Earthworm - including Morphology and anatomy: Definition, Types, Characteristics, Classification, Examples

Definition of Earthworms

Earthworms have a significant function of casting fresh air to the bottom of the soil and mixing it. This provides suitable conditions for plant growth. This article presents the biology of earthworms and their contribution to agro and natural ecosystems, as well as the applications of the earthworm.

Soil segmentation and subsequent burial of organic matter by earthworms are also considered to be important for soil fertility and health. These animals are beneficial due to their ability to decompose organic matter, which, in doing so, improves the structure, nutritional content and water-holding capacities of soils.

Also Read:

• Animal Kingdom
• Phylum Platyhelminthes
• Phylum Ctenophora

Taxonomy And Classification of Earthworm

Earthworms are in the phylum Annelida, which comprises segmented worms. They are in the class Oligochaeta, characterised by a few bristles on the segments of their bodies. Earthworms are in the order Haplotaxida and the family Lumbricidae, which have numerous common species occurring in soil. The best-represented genus is Lumbricus, of which the most common species is Lumbricus terrestris. This classification indicates their body being segmented, burrowing nature, and key position in soil communities.

• Kingdom: Animalia

• Phylum: Annelida

• Class: Clitellata

• Order: Opisthopora

• Family: Lumbricidae

Anatomy And Morphology of Earthworm

Earthworms have long, cylindrical, and segmented bodies that are soft and wet to help with free movement in the soil. The body has bilateral symmetry and is guarded by a thin cuticle to shield it. The body of an earthworm consists of segments referred to as metameres, and some of the segments have specialised structures such as the clitellum, which facilitates reproduction. Within, they have a simple yet effective system for digestion, circulation, and excretion that has evolved to accommodate their burrowing habit.

• Earthworms possess a tube-shaped and long body composed of numerous segments.
• The body is bilaterally symmetrical, with the left and right sides being the same.
• The outer layer is a thin cuticle that maintains the skin moist and guarded.
• They consist of metameres (segments) that provide flexibility to the body.
• There is a thickened band known as the clitellum found on mature earthworms.
• There are minute bristles known as setae found on every segment to help movement.
• Their mouth is at the front (anterior) end, and their anus is at the back (posterior end).
• Internally, they have a rudimentary organ system for burrowing and consuming soil

A diagram of an earthworm

Digestive System of the Earthworm

The digestive system of earthworms starts with the mouth through which the worm takes in soil with decomposable substances. Digestion then proceeds to the pharynx and the oesophagus, and following this, the crop, where it is accumulated for a while. After that, the food goes to the gizzard to be mechanically crushed and broken into smaller pieces. Last, the chewed food moves to the intestine, where substances in the food are taken in and out through the rectum.

Diagram of the digestive system of the Earthworm

Circulatory System of Earthworm

The circulatory system of earthworms is a closed type, implying that the blood is enclosed in vessels. The first important vessels are the blood vessels located dorsally and ventrally along the length of the body. Another recent structure of the worm’s circulatory system consists of several pairs of hearts, locally named aortic arches, which pump blood through these vessels and very effectively transfer nutrients and gases.

Diagram of the circulatory system of the Earthworm

Nervous System of the Earthworm

The basic structure of an earthworm’s nervous system is the brain situated at the anterior part of the worm and a ventral nerve cord that runs spinal throughout the length of the worm. On this nerve cord, the segmental ganglia are in every segment to coordinate the local functions and movements. This structure helps the earthworm to take appropriate action as far as environmental factors are concerned.

Excretory System of the Earthworm

Oligochaetes eliminate the waste products through other structures called nephridia, which are located in nearly every part of the body of the oligochaete. These nephridia help filter metabolic waste from the blood and coelomic fluid out of the body via pores on the body’s external surface of the worm. This system aids in the regulation of the chest condition, or rather, the internal chemical condition of the worm.

Reproductive System of the Earthworm

The worms that inhabit the soil are earthworms, and interesting that these worms are hermaphrodites, meaning that they have both male and female organs. Lastly, reproduction for the earthworms, two of them copulating, laying their ventral sides closer to each other to swap sperm. The clitellum next secretes a mucus ring to cover the spermated eggs to facilitate the development of protective cocoons around their eggs.

Diagram of the Reproductive System of the Earthworm

Physiology And Biological Functions of Earthworm

The physiology of earthworms is simple but highly adapted to soil life. Earthworms respire through their wet skin in where gas exchange takes place by diffusion. Oxygen and nutrients are carried to every region of the body through their closed circulatory system. The digestive system helps them to digest organic material in soil, whereas the muscular system helps them in burrowing and locomotion. Earthworms have a simple nervous system with a nerve cord and basic sense organs to feel light, touch, and vibrations, which helps them survive in the ground. Some important points are discussed below:

Habitat And Distribution of Earthworm

Earthworms are found in broad distribution in rich organic soils with high water content throughout the globe. They are adapted to live in conditions where the soil is loose, soft, and highly decomposed plant material, as burrowing and feeding are easy in such conditions. Earthworms dominate gardens, fields, forests, and agricultural lands where soil remains wet and well-aerated. Earthworms do not like dry, sandy, or waterlogged soils because they require water for respiration and living. Their appearance is frequently an indicator of fertile, healthy soil.

• Earthworms inhabit rich, moist soils with plenty of organic matter.
• They are widespread in gardens, fields, and forests.
• They are well-suited to loose and soft earth that permits effortless burrowing.
• Earthworms occur on every continent except Antarctica.
• They are averse to dry, sandy, or rocky soil because it restricts movement.
• They do not live easily in waterlogged or flooded areas.
• Earthworms are more active in temperate and tropical areas.
• They are distributed in a way that maintains soil fertility and structure.

Earthworms' Role in the Ecosystem

Earthworms are a very important component of the ecosystem since they improve soil structure, aeration, and fertility through burrowing and feeding. As they move through the soil, they provide pathways for water, air, and the roots of plants to extend deeper into the soil. Through organic matter decomposition and the production of nutrient-dense casts, they facilitate nutrient cycling and plant-mineral accessibility. Their existence assists in keeping soils healthy, which is important for plant growth, farming, and general ecosystem health.

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