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Earthworms

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The Earthworm (Lumbricus terrestris)

Phylum: Annelida (segmented worms)
Characteristics:
  • Bilateral symmetry
  • Triploblastic coelomate
  • Body segmented internally and externally
  • Setae Circulatory system
  • Mouth and anus
  • Nephridia are the organs for the excretion of nitrogenous waste.

    External structure


  • When fully grown it is about 17cm long and has about 150 segments.
  • The body is cylindrical (but flattened slightly towards the posterior end)
  • The upper(dorsal) surface is dark red in colour and the lower (ventral) surface is lighter.
  • Externally the segments are seen as small rings along the body.
  • Internally each segment is separated from the next by a partition called a septum.
  • The body is covered in a thin, transparent cuticle which helps to prevent it from drying out. This cuticle is covered in mucus which helps in exchange of gases.
  • The mouth is in the first segment and the anus is in the last. There is a fleshy lobe-like structure called the prostomium that hangs over the mouth.
  • Sexually mature worms have a swelling called the clitellum between segments 32 and 37.

    T.S. Earthworm


    Diet
    Earthworms feed on detritus (i.e. small pieces of dead and decomposing plants and animals). They mainly feed on dead and decaying leaves.

    The alimentary canal


    Prostomium: Tastes and grips the food.
    Pharynx: Pulls the food into the alimentary canal.
    Oesophagus: Passes the food into the crop.
    Crop: Stores and softens the food.
    Gizzard: Mechanical digestion of food using pebbles and pieces of grit.
    Intestine: Enzymes are released to chemically breakdown the food and the soluble pieces areabsorbed. There is an infolding on the dorsal side of the wall of the intestine which increases the surface area for absorption of food. This is called the typhlosole.
    Anus: The undigested food and soil is egested through the anus as 'worm casts'.

    Vascular system
    As the earthworm is triploblastic coelomate transfer of materials by diffusion would not be adequate. Therefore, a specialised transport system is essential.
    The earthworm has a closed system of blood vessels and a single circulation.
    The major blood vessels are:
    1. Ventral blood vessel: Below the alimentary canal, distributes the blood to various parts of the body and carries it towards the posterior end.
    2. Branches of the ventral blood vessel: Take the blood to all the structures within each segment. These branches form capillaries, which return blood to the dorsal blood vessel.
    3. Dorsal blood vessel: Above the alimentary canal, collects the blood from the body and carries it towards the anterior end.
    4. Sub neural blood vessel: Below the nerve cord, drains the blood from the nerve cord and carries it towards the posterior end. In segments 7 - 11 there are five pairs of side vessels connecting the dorsal and the ventral blood vessels. These can contract and relax and are called pseudohearts (false hearts). They contract to pump the blood. Each heart has four valves which allows the blood to flow in a downward direction only and prevent an upwards flow.

    The Nervous system
    Earthworms do not have visible sense organs. Receptors sensitive to light, touch and chemicals are present along the body.
    The cerebral ganglia or 'brain' lies above the pharynx. This is connected by a nerve collar to the ventral ganglion which lies below the pharynx. A nerve cord extends back along the body from the ventral ganglion. In each segment the nerve cord swells slightly to form a segmental ganglion from which three pairs of nerves branch off to supply all parts of the segment.

    Movement
    It involves the circular muscles, longitudinal muscles and setae. Each segment except for the first and the last has four pairs of setae. The setae help movement by gripping the soil.The circular muscles contract which makes part or all of the body becomes longer and thinner.
    The longitudinal muscles contract which makes part or all of the body becomes shorter. The alternate lengthening and shortening of the body along with the correct positioning of the setae is what brings about movement.

    Excretion
    The main excretory organs are the nephridia. A pair of nephridia are found in each segment except the first three and the last one. A nephridium consists of a ciliated funnel the nephristome, which connects, by a series of thin coiled tubes and a larger muscular tube, to a pore on the surface called a nephridiopore. The tube is coiled to provide a greater surface area for reabsorption of useful materials.

    Reproduction
    Earthworms are hermaphrodite. The reproductive parts are located in segments 9 to 15, and the clitellum is from segment 32 to 37.
    Male reproductive organs
  • A pair of testes are found in each of segments 10 and 11. They produce the sperm.
  • A pair of spermathecae are found in each of segment 9 and 10. They store the sperm from the other worm during copulation.
  • Seminal vesicles are found between segments 9 and 12. These store the sperm and produce a fluid in which the sperm can swim.
  • Sperm ducts carry the sperm to the outside of the body at segment 15. Female reproductive organs
  • A pair of ovaries is found in segment 13. They produce the eggs.
  • A pair of ovisacs also found in segment 13 collects and stores the eggs.
  • A pair of oviducts carries the eggs to segment 14 where they are released from the body.
    Copulation
  • It occurs at night on the surface of the soil.
  • The two worms face in opposite directions and their ventral surfaces are in contact.
  • The clitellum of each worm (segment 32 to 37) is opposite segments 9 to 15 of the other.
  • The worms are held in this position by setae and by mucus sheaths secreted by each clitellum.
  • The sperm are released by each worm from segment 15, and are shed into the two seminal grooves. The sperm travel along the grooves to the clitellum.
  • The sperm then exit from the sheath and cross to the spermathecae of the other worm at segments 9 and 10 where they are stored.
  • Copulation takes a few hours and then the worms separate and return to the soil.
    Cocoon formation
  • A few days after copulation the clitellum of each earthworm secretes a tough chitin-like tube (the cocoon).
  • The cocoon slides forward towards the anterior end of the worm.
  • As it slides forward it collects eggs at segment 14 and sperm from the spermathecae at segments 9 and 10.
  • Fertilisation occurs in the cocoon.
  • The cocoon now containing zygotes is pushed off and set free onto the soil. It seals at both ends.
  • Only one worm emerges from the cocoon a few weeks later.

    The importance of earthworms in agriculture
    1. Their burrows increase soil drainage.
    2. Their burrows increase soil aeration.
    3. They soften the soil allowing easier root penetration.
    4. They speed up the formation of humus.
    5. They mix soil layers.
    6. Increase the depth of topsoil by bringing subsoil to the surface.
    7. The passage of soil through their alimentary canal results in the particles of soil being ground to a finer texture in the gizzard of the worm.
    8. Their dead bodies become part of humus.
    9. They reduce the acidity of the soil.

    Experiment
    Title: To observe the activities of earthworms.
    1. Place alternate layers of gravel, sand, peat, soil and chalk in a wormery.
    2. Place a few leaves on the surface (as a food supply). Keep it moist.
    3. Make a drawing of the layers.
    4. Add a few earthworms.
    5. Cover the glass slides with a black cloth and leave it for a few days.
    6. Remove the black cloth and observe.
    Results:
    (a) The soil layers will be mixed.
    (b) Burrows will be visible in the soil.
    (c) The plant material will be gone from the top.
    (d) The soil level will have increased.

    Experiment
    Title: Dissection of an earthworm.
    1. Place the earthworm on a wax dish with the dorsal surface uppermost (i.e. the dark, smooth side).
    2. Pin the earthworm down making sure it is fully stretched.
    3. Using a scissors make an incision just behind the clitellum.
    4. Cut forwards beside the dorsal blood vessel. Keep the scissors pointed upwards.
    5. Break the septa with pins and pin back the body wall. Insert the pins at a slant.
    6. Cover the dissection with water.
    7. Identify and flag label all the parts.

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