7.4 Earthworm:
An earthworm is a tube shaped segmented worm and reddish brown terrestrial invertebrate found in the phylum Annelida.
The common Indian earthworms are Pheretima and Lumbricus.
Scientific name of Earthworm: Pheretima posthuma
7.4.1 Habit and Habitat:
It lives in the upper layer of moist soil.
During the day, they live in burrows (holes) made by boring the soil.
It feeds on dead and decaying organic matter along with soil.
The fecal deposits of earthworms are known as Worm Castings. Because of fecal deposits, earthworms can easily be traced.
7.4.2 Morphology of Earthworm:
Earthworm has a long cylindrical body which is divided into more than a 100 short segments or metameres.
Dorsal Surface: There is a dark median mid-dorsal line; along the longitudinal axis; on the dorsal surface of the body. This line marks the dorsal blood vessel.
Ventral Surface: It is a light body surface due to the absence of blood vessels. It contains genital apertures.
Numerous nephridial pores on the skin of each segment helps in excretion.
Body of the earthworm is divided into three regions based on the presence of clitellum.
Preclitellar
Clitellar and
Postclitellar.
7.4.2.1 Preclitellar:
This region is present above the clitellum (anterior region).
The fleshy lobe, overhanging the mouth (front of the mouth) is called Prostomium.
Peristomium contains Prostomium and Mouth openings. It is the first segment of the anterior end in earthworms.
In this region, four pairs of spermathecal apertures are situated on the ventro-lateral sides of the segments from 6th to 9th.
7.4.2.2 Clitellar:
In a mature worm, segments 14-16 are covered by a prominent dark band of glandular tissue. This band is called clitellum.Savinay Kumar JC
This region contain external genital apertures;
A female genital aperture in the 14th segment situated centrally.
A pair of male genital apertures in the 18th segment ventro-laterally.
Above and below the male genital aperture can observe Genital Papillae.
7.4.2.3 Post clitellar: Observe Anus at the end of last segment in earthworm.
7.4.3 Body wall of Earthworm:
Body wall is externally covered by a thin acellular cuticle.
Body wall contains epidermis below the cuticle.Savinay Kumar JC
Epidermis is made of a single layer of columnar epithelium with secretary glands.
Two layers of muscles (circular and longitudinal) helps in locomotion.
7.4.4 Alimentary Canal of Earthworm:
It consists of Mouth, Pharynx, Esophagus, Gizzard, Stomach, Intestine and Anus.
Mouth, Pharynx, Esophagus, and Gizzard helpful for grinding of food like solid particles and decaying leaves.
Stomach contains calciferous glands, this gland helps in neutralizing humic acid and digestion.
The Intestine of earthworm starts from the 15th segment and continues till the last segment.Savinay Kumar JC
Intestine is further divided into typhlosole, intestinal caecum, Lymph gland, and Intestinal lumen.
Typhlosole increases an effective area of absorption in the intestine.
Intestinal caecum helps in mixing, absorption of fluids and salts. It is in a pair of short and canonical forms.
Lymph Gland secretes digestive enzymes.Savinay Kumar JC
Intestinal lumen is the space provided to pass food.
Anus is helpful in removing waste and undigested food from earthworms outside.
7.4.5 Locomotion in Earthworm:
There are 5 rows of S-shaped setae in each body segment.
The setae are embedded in the epidermal pits in the middle of each segment.
Setae are absent in the first segment, clitellum and last segment.
Setae can be extended or retracted and facilitate locomotion.
7.4.6 Closed Circulatory System of Earthworm:
It consists of the blood vessels, hearts, loops and the blood glands.
Due to a closed circulatory system, blood is confined to the heart and blood vessels.
Contractions keep blood circulating in one direction.
Blood Vessels are divided into two types;
Collecting blood vessels (dorsal vessels)
Distributing blood vessels (ventral vessels)Savinay Kumar JC
Dorsal Vessels collect deoxygenated blood from different parts of the body through capillaries.
Ventral vessels distribute oxygenated blood to different parts of the body through capillaries.
Commissural vessel receives blood from the body wall of the septum.
Subneural vessel is a thin vessel. It receives blood from the nerve cord.
Lateral esophageal vessels receive and distribute blood to alimentary canal walls.
Four pairs of hearts are present. Helpful for pumping.Savinay Kumar JC
Anterior loops help in collecting blood from ventral vessels to the heart.
Blood glands are helpful in synthesis of blood and hemoglobin (no RBC).
Hemoglobin dissolved in blood plasma.
Blood cells are Phagocytic in nature.
7.4.7 Respiration:
Earthworms lack of specialized breathing devices.Savinay Kumar JC
Gaseous exchange takes place through the moist surface of skin.
Transport of respiratory gasses takes place in hemoglobin.
7.4.8 Excretory System in Earthworm:
Excretory Organ of an earthworm is Nephridia. This helps to remove nitrogenous waste from the body.
Nephridia of Earthworm is divided into three types as follows;
Septal Nephridia
Integumentary Nephridia and
Pharyngeal Nephridia
Septal Nephridia: It is attached to the septum of earthworm. It starts from the 15th segment to the last segment. It removes nitrogenous waste through anus.
Integumentary Nephridia: It is present on an inner surface of the body wall. It removes nitrogenous waste through skin.
Pharyngeal Nephridia: It is located in the anterior side of the pharynx. It is further divided into two types as follows;
Tufts of pharyngeal Nephridia: It collects nitrogenous waste in the buccal cavity and sends it to the ducts of pharyngeal nephridia.
Ducts of pharyngeal Nephridia: It sends nitrogenous waste to the alimentary canal and removes through anus.
Note:Savinay Kumar JC
Nephridia regulates the volume and composition of body fluids.
A nephridium starts with a ciliated funnel-like structure called nephrostome, connected with a tubular part.
7.4.9 Nervous system of Earthworm:
Represented by ganglia arranged segmentally on the ventral paired nerve cord.Savinay Kumar JC
Nerve cord in the anterior region (3rd and 4th segments) bifurcates, laterally encircling the pharynx and joins the cerebral ganglia dorsally to form nerve rings.
The cerebral ganglia along with other nerves in the ring integrate sensory input as well as command muscular responses of the body.
7.4.10 Sensory organs:
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) which react to chemical stimuli. These sense organs are located on the anterior part of the worm.
7.4.11 Reproductive System of Earthworm:
Earthworm is hermaphrodite (bisexual) i.e., testes and ovaries are present in the same individual.
It consist four pairs of spermatheca, Testis sac, Testis, Spermidual Funnels, Seminal Vesicles, Ovary, Ovarian Funnel, Vasa Deferentia, Accessory glands, Prostate gland, and common Protatic & Spermatic duct.
Four pairs of spermathecae are located in 6th – 9th segments.They receive and store sperm during copulation.
Testis sac, Testis, Spermidual Funnels, and Seminal Vesicles helps in production of sperm.Savinay Kumar JC
Ovary, and Ovarian Funnel, helps in production of egg/ovum.
Vasa Deferentia are helpful in transport of sperm from testis to spermatic duct.
From a spermatic duct, sperm releases into the female genital pore.
Accessory glands secrete fluid. This fluid mixes with the sperm to protect and preserve them.
Prostate gland secrets prostate fluid. It helps in nourishment and protects sperm.
7.4.12 Fertilization:Savinay Kumar JC
Even though earthworm is a bisexual organism, it still needs for sexual reproduction another earthworm for mating.
At the time of mating both earthworms in Juxtaposing.
A mass or packet of sperm releasing during mating called spermatophores.
Fertilization is internal.
Fertilization and development takes place inside the cocoon which is deposited in soil.
Earthworm cocoon is a sac containing the earthworm eggs.
After three weeks each cocoon produces two to twenty baby worms.
Development is direct without a larval stage.
It is a protandrous animal (individual functions first as a male and later as a female; protogyny is a reverse situation) with cross fertilization. Savinay Kumar JC
7.4.13 Economic Importance:
Earthworms are called ‘Friends of Farmers’.
They burrow in the soil and make it porous. It helps in respiration and penetration of developing plant roots.
The process of increasing fertility of soil by earthworms through fecal castings is called Vermi-composting.
Earthworms are also used as bait in fishing.
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