Characteristics of Earthworms: Habits, Habitats, Morphology, Body Wall, and Locomotion.

Table of Contents

• Habit and Habitat of Earthworm
• A. Shape and size
• B. Color
• C. Segmentation
• D. Head
• E. Clitellum
• F. Setae
• G. External Apertures of Earthworm
• 1. Mouth
• 2. Anus
• 3. Genital openings
• 4. Spermathecal pores
• 5. Nephridiopores
• 6. Dorsal pores
• 7. Genital papillae
• H. Body wall of Earthworm
• 1. Cuticle
• 2. Epidermis
• 3. Muscular Layer
• 4. Coelomic epithelium
• The function of the body wall of Earthworm
• Locomotion of Earthworm

Habit and Habitat of Earthworm

• Earthworms are burrowing creatures, displaying a preference for residing in their burrows during daylight hours, while emerging at night or during damp, cloudy weather to seek food, explore new habitats, and engage in reproductive activities. They exhibit a nocturnal lifestyle.
• The rainy season proves to be highly favorable for earthworms, as they tend to leave their burrows and become visible in substantial numbers on the ground after heavy rainfall. They take advantage of this period for breeding purposes.
• These creatures are commonly found in soil abundant with decaying organic matter, typically in gardens, pastures, lawns, irrigated farmlands, and near the banks of ponds, lakes, and rivers.
• As cold-blooded or poikilothermal organisms, their body temperatures fluctuate with the surrounding environmental conditions.
• Earthworms sustain themselves by consuming dead organic matter. They ingest both food and soil together, subsequently expelling the latter, along with undigested food, in the form of nutrient-rich worm castings.
• While being hermaphrodites, earthworms engage in copulation to exchange spermatozoa. Their fertilization and development processes take place within protective cocoons.
• The natural lifespan of earthworms varies, lasting from 3 and a half years to approximately 10 years.

External Morphology of Earthworm

The external features or external morphology of an Earthworm are characterized as follows:

A. Shape and size

• The Earthworm displays a long, narrow, and cylindrically elongated body with a pointed front and a blunt rear, being thickest slightly behind the anterior end.
• It exhibits bilateral symmetry, meaning its left and right sides are mirror images of each other.
• On the dorsal surface of the body, there is a noticeable dark median line formed by a dorsal blood vessel that extends throughout the body just below the skin.
• The ventral surface is marked by genital openings and papillae in the anterior sections of the body.
• The size of Earthworms can vary among different species and even among individuals of the same species.
• Typically, a mature Earthworm measures about 150mm in length and has a width ranging from 3 to 5 mm.

B. Color

• When touched, the Earthworm's body feels slimy and appears glistening with a color ranging from deep brown to clay.
• This coloration is attributed to the presence of the pigment porphyrin in its body wall, which serves to protect the organism against intense and bright light.
• The dorsal surface of the Earthworm is darker in comparison to its ventral surface.

C. Segmentation

• The Earthworm's body is comprised of approximately 100-120 small ring-like segments, also known as somites or metameres.
• True segmentation is a prominent characteristic of Earthworms.
• These segments lack parapodia, which are the fleshy appendages found in some other segmented organisms.
• Distinct ring-like grooves or furrows separate each segment from one another, signifying their external segmentation.
• Internally, the body is divided by intersegmental septa or coelosepta into small chambers, representing internal segmentation.
• Except for the first and last segments, all other segments of the Earthworm's body are similar in structure and appearance.

D. Head

• The Earthworm lacks a distinct head and any well-defined sense organs like eyes, cirri, or tentacles.
• At the anterior end of the body, the first segment is called the buccal segment or peristomium.
• The peristomium bears a crescent-shaped mouth, and it extends anteriorly into a fleshy lobe known as the prostomium, which overhangs the mouth.
• The prostomium, often called the boring part, helps the Earthworm in burrowing through the soil.

E. Clitellum

• In mature earthworms, the 14th, 15th, and 16th segments are enclosed by a thick-collar or girdle-like glandular tissue known as the clitellum.
• Cocoon formation takes place within the clitellum during the reproductive process.
• The presence of the clitellum indicates the maturity of the worm, as it is absent in immature individuals.
• The clitellum is a glandular organ responsible for secreting mucus, albumen, and materials necessary for forming cocoons, which aid in fertilization as the eggs are laid within them.

The body of an Earthworm is divided into three regions:

1. Pre-clitellar region:

• Comprises segments 1 to 13, with the first segment being the peristomium that surrounds the mouth.
• At the anterior end of the peristomium, there is a small fleshy lobe called the prostomium.

2. Clitellar region: Segments 14, 15, and 16, where the clitellum is located.

3. Post-clitellar region: 

• Comprising segments 17 and beyond, extending towards the posterior region.
• The clitellum is sometimes referred to as the "Forest of Nephridia" due to its role in reproductive and excretory functions.

F. Setae

• With the exception of the first, last, and clitellum-bearing segments, all other segments of the Earthworm's body are equipped with chitinous setae.
• These setae lie embedded in the middle of each segment and project backwardly.
• Approximately 80-120 setae are present on each segment.
•  Each seta is small, elongated, and S-shaped, with a faint yellow color.
• A seta consists of three parts: the neck, the swollen middle part called the modulus, and the root or body, which attaches to a setal sac or setigerous, along with muscles.
• Special muscles control the movement of setae, allowing them to be extended or withdrawn in any direction at the will of the Earthworm.
• Setae serve as the locomotory organs of the Earthworm, enabling it to move through the soil.
• Immature worms possess setae on the clitellar segment, but they shed them before reaching maturity.
• Setae are made of a horny nitrogenous organic substance called chitin.
• They are arranged in an annular row on the mid-ventral surface of each segment, a configuration known as perichaetine arrangement.

G. External Apertures of Earthworm

1. Mouth

• The Earthworm's mouth is a crescent-shaped anterior aperture.
• It is located just below the prostomium on the ventral side.
• The first segment of the body, known as the peristomium or buccal segment, surrounds the mouth.

2. Anus

• The anus is a vertical slit-like aperture situated at the posterior terminus.
• It serves as the exit for the alimentary canal, eliminating undigested wastes.

3. Genital openings

• Earthworms are hermaphrodites, possessing both male and female generative apertures in the same individual.

a. Male genital aperture

• Located latero-ventrally in the 18th segment.
• Consists of a pair of crescentic openings of common prostatic and spermatic ducts.

b. Female genital aperture

• Positioned at the ventral side in the 14th segment within the clitellar region.
• This is where female reproductive bodies are discharged from.

4. Spermathecal pores

• Four pairs of small ventrolateral spermathecal apertures.
• Found between segment grooves 5/6, 6/7, 7/8, and 8/9.

5. Nephridiopores

• Numerous tiny nephridiopores scattered throughout the body, except in the first two segments.
• These pores serve as the external openings of the integumentary nephridia, through which metabolic wastes are removed from the body.

6. Dorsal pores

• Minute apertures of coelomic chambers.
• Located mid-dorsally, one in each intersegmental groove, behind the 12th segment, except for the last groove.
• These pores facilitate communication between the coelom and the exterior.

7. Genital papillae

• Earthworms have pairs of conspicuous rounded elevations or circular raised papillae located in line with the male pores.
• These pairs of papillae are found on the ventral side of each of the 17th and 19th segments.
• Each genital papilla features a cup-like depression at the top but lacks any aperture.
• These papillae function as suckers during copulation.

H. Body Wall of Earthworm

The body wall of an Earthworm is thin, soft, shiny, elastic, and highly vascular. It consists of:

1. Cuticle

• The cuticle is a thin, elastic, non-cellular, and finely striated layer.
• It is secreted by the columnar epithelial or supporting cells of the underlying epidermis.
• The cuticle is perforated by minute pores, through which integumentary nephridial and epidermal glands open out.
• This layer provides protection to the Earthworm's body.
• It is composed of two layers of collagenous proteins forming fibers and a polysaccharide with a small amount of gelatin.

2. Epidermis

The epidermis is a single-layered but multicellular tissue located just below the cuticle.

It consists of tall, cylindrical epidermal cells with various functions:

a. Supporting cells: Long columnar cells that make up a major part of the epidermis and have an oval nucleus in the middle.

b. Gland cells: Thicker cells with numerous mucous cells and a few albumen cells containing secretory granules. Mucous cells secrete mucus to keep the body moist, assist in locomotion, and prevent desiccation. Albumen cells secrete albumen.

c. Basal cells: Small conical cells lying between supporting cells and gland cells, which can differentiate into supporting or gland cells.

d. Sensory receptor cells: Narrow, columnar cells grouped together, containing hair-like processes at their outer free ends and functioning as sensory receptors.

3. Muscular Layer

• The muscular layer is situated below the epidermis.
• It consists of an outer thin layer of circular muscle fibers encircling the body and an inner longitudinal muscle fiber running along the length of the body.
• The longitudinal muscle fibers lie in parallel bundles, separated by connective tissue and strengthened by collagen fibers.
• Contraction of the circular muscles makes the body long and narrow, while contraction of longitudinal muscles makes it short and broad.
• The musculature of the body wall consists of smooth muscle fibers.
• The outer circular muscles contain numerous scattered granules of porphyrin pigment.
• Two additional muscles are found at the base of each setal sac: a pair of protractor muscles and a single retractor muscle.

4. Coelomic Epithelium

• The coelomic epithelium, also known as the parietal peritoneum or parietal layer, is the outer covering of the coelomic cavity.
• It consists of flat cells recognized by their nuclei.

Functions of the Earthworm's Body Wall:

• Provides shape and elasticity to the body.
• Mucus secreted by the gland cells of the epidermis keeps the body smooth and moist, aiding in respiration and locomotion.
• Protects internal delicate organs from injury.
• Secretes mucus that aids in nutrition and locomotion.
• Sensory epidermal cells act as receptors for receiving external stimuli.
• The coelomic epithelium secretes coelomic fluid.
• The cuticle prevents excessive evaporation.

Locomotion of Earthworms:

• Earthworms lack specialized locomotory organs but are highly active and can crawl rapidly when outside their burrows.
•  Movements involve a combination of muscular contractions in the body wall, setae, and hydrostatic pressure generated by the coelomic fluid.
• Increased hydrostatic pressure in the anterior segments (usually nine segments) contributes to forward locomotion.
• Contraction of circular muscles starts at the anterior end and propagates backward as a wave, causing the anterior region to extend forward and become thinner in diameter.
• The extending and thinning process repeats, pushing the worm forward.
• Once the anterior end grips the substrate, the setae act as hooks with their posteriorly directed points.
• A wave of longitudinal muscle contraction causes the body to thicken and shorten, while a wave of thinning follows, moving the segments forward.
• Each wave of circular contraction propels the affected segments forward.
• During locomotion, coelomic fluid acts as a hydraulic skeleton, as its pressure influences muscle contraction and relaxation.
• Earthworms can move backward by reversing the direction of the waves.
• On a smooth and hard surface like glass, earthworms use mucus for adhesion since the setae cannot anchor them to such substrates.