Earthworm: Morphology & Anatomy

An earthworm can be referred to as a segmented worm that belongs to the phylum Annelida. The terrestrial invertebrates are usually found in moist soil and feed on organic matter.

• The farmer's companion, the earthworm, is an annelid that aids in soil fertility retention.
• This is an invertebrate animal that lives primarily in the soil's upper layer.
• Worm castings are the faecal deposits of the earthworm.
• If you see these worm castings in your garden soil, you can almost certainly find a few earthworms if you dig into them.
• Earthworms are classified as a genus by the scientific name Pheretima.
• Mouth, anus, pharynx, crop (which stores food), gizzard (filled with small stones) and intestine are important body parts of earthworms.

Key Terms: Earthworm, Earthworm Diagram, Anatomy of Earthworm, Morphology, Alimentary canal, Closed Circulatory System, Reproductive System

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Earthworm Diagram

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The earthworm diagram depicts the morphological features of the given species.

Earthworm Diagram 

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Morphology of Earthworm

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Earthworms are reddish-brown in colour and have a cylindrical body divided into segments called metamers. The body is also elongated and pointed in the anterior region, with a rounded posterior region. 

• Earthworm's body is segmented, with approximately 100 to 120 metameres, or short segments. 
• On the dorsal surface of the body, there is a dark median mid-dorsal blood vessel. 
• Genital openings or pores can be found on the body's ventral surface.
• The peristomium is the first body segment with a mouth.
• The anus can be found in the final segment. 
• The clitellum is a prominent dark band of glandular tissue located between segments 14 and 16. 
• These are the segments that have been joined together. 
• Four pairs of spermathecal apertures are located between the 5th, 6th, 7th, 8th, and 9th intersegments. 
• A single female genital pore is present on the 14th segment, in the midventral line.
• On the 18th segment, there is a single pair of male genital pores. 
• On the body's surface, there are numerous minute pores known as nephridiopores. 
• Except for the first, last, and clitellum segments, all other segments have embedded S-shaped setae. 
• These setae aid in locomotion.

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Anatomy of Earthworm

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The earthworm's body is covered by a thin layer of non-cellular cuticle, two muscle layers, and the coelomic epithelium. Coelomic epithelium is the inner layer, which is sheathed above an epidermis layer and the cuticle on top. The epithelium is made up of glandular columnar that exists in a single layer.

Reproductive System 

The reproductive system of an earthworm is monoecious or hermaphrodite, which means it has both male and female reproductive organs. However, self-fertilization does not occur because the male and female reproductive organs do not mature at the same time. 

• Protandry is a condition in which the testes mature before the ovaries. 
• As a result, cross-fertilization occurs, which is followed by cocoon formation.

Components of Male Reproductive System

The following are the components of the male reproductive system:

• Two pairs of the Testis: One pair of the testis is held in each of the tenth and eleventh segments.
• Vasa Deferentia: There are two pairs of vasa deferentia. Each side of vasa deferentia runs close to each other until the 18th segment. The 18th segment connects both vasa differentia from each side to the prostate duct coming from the prostate gland.
• Accessory Glands: Two pairs of accessory glands are located internally in the 17th and 19th segments. During copulation, the secretion of these glands aids in the retention of the two worms.

Components of Female Reproductive System

The female reproductive system consists of: 

• One Ovary and one Oviduct: A pair of white minute ovary masses are present between the 12th and 13th segments to the posterior surface of the septum. Behind the ovaries, there are two short tubes, each of which lays eggs.
• Four pairs of Spermathecae: In the 6th and 9th segments, where the spermatozoa are stored.

Circulatory System 

The earthworms have a closed blood vascular system, which means that blood is confined to the heart and blood vessels.

Heart

There are four pairs of tubular hearts in an earthworm. The 7th and 9th segments contain the lateral hearts, which are the two anterior pairs of hearts. 

• They connect the dorsal and ventral blood vessels. 
• The posterior two pairs of hearts, known as latero-oesophageal hearts, are present in the 12th and 13th segments.

Blood Vessels

There are two types of blood vessels found in earthworms namely Dorsal Blood Vessel and Ventral Blood Vessels. Dorsal Blood Vessel is the largest blood vessel in the body, running mid-dorsally above the alimentary canal. 

• The valves in the vessel prevent blood from flowing backwards. 
• Its function is to receive blood from various parts of the earthworm via the various connecting vessels.
• Ventral Blood Vessels do not include any valves. 
• The blood flows from the front to the back of the body. 
• Its primary function is to distribute blood.

Blood

The red colour of an earthworm's blood comes from a protein called haemoglobin, which is found in the plasma and aids in the transportation of oxygen for respiration.

• The blood is made up of two components: fluid plasma and colourless blood corpuscles.
• The only type of blood corpuscles found in the blood of an earthworm are leukocytes, which are phagocytic in nature and enter the body.

Blood Glands

From the fourth to sixth segments, small red-coloured follicular bodies are found, which are referred to as blood glands.

Respiratory System

Earthworms do not have a well-developed structure for respiration. The respiration is done through the moist skin by the process of diffusion.

Excretory System

The excretory system of earthworms consists of nephridium, which is a coiled tubule responsible for regulating the volume and composition of body fluids. 

• Nephridia is arranged in three segments: septal, integumentary, and pharyngeal nephridia.
• Septal has 15-last segments, integumentary has 3-last segments, and pharyngeal nephridia has 4-6 segments.
• It is connected to a funnel for delivering wastes and excess fluid and is later excreted out via the digestive tube.

Nervous System

The nervous system of earthworms consists of sensory input and muscular responses. The muscular responses and sensory input are responsible for controlling ganglia, which are paired with the nerve cord to form the nervous system of earthworms.

Sensory System

The sensory system of earthworms is located in the anterior portion of the body. Though earthworms do not have eyes, they have specialized receptor cells that recognise changes taking place in the environment. 

• The response to stimuli is controlled by specialised sensory organs and chemoreceptors.

Digestive System

Earthworms are soil-dwelling creatures whose alimentary canal is in the shape of a long tube running from the first to the last segment of the body. The food of earthworms is mixed with soil.

• The gastrointestinal tract is a straight tube that runs from the mouth to the anus. 
• The peristomium, also known as the buccal segment, is the first body segment that includes the mouth.
• The pharynx is a pear-shaped chamber that starts from the 3rd segment and continues to the 4th segment. 
• The gizzard acts as a grinder, thus helping earthworms in the grinding of soil particles combined with organic matter and other decaying materials. 

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Things to Remember

• An earthworms feed on organic matter and are common inhabitants of moist soil.
• The dorsal side is distinguished by a dark line of blood vessels, while genital openings distinguish the ventral side. 
• The anterior end is distinguished by the mouth and the prostomium (an organ that aids in burrowing).
• The earthworm's body wall is covered by a thin, non-cellular cuticle. 
• A layer of the epidermis is sheathed beneath the cuticle, followed by two muscle layers and coelomic epithelium (inner layer).
• A single layer of glandular columnar epithelium makes up the epithelium.