EARTHWORM: HABITS & EXTERNAL MORPHOLOGY

ByShrim Gautam •
· The generic name Pheretima was first coined by Kinberg (1867 AD).
· The generic name Pheretima has been revised to Metaphire by Sims and Easton (1972 AD).
· Earthworm is usually studied as a type of Annelida because it is easily available, harmless to handle, size is convenient and has well-known natural history and morphology.


Systematic position of Earthworm:
Kingdom: Animalia
 Phylum: Annelida
  Class: Oligochaeta (L. Oligos, few + Chaete, bristles)
   Family: Megascolecidae
    Genus: Pheretima or Metaphire
     Species: posthuma


Habits

The earthworm has the following habits.

1. Feeding
· It is detritivores (feed upon decaying organic matter) as well as an omnivore (feed on bits of plants and animal matter)
· It takes soil by the sucking action of the pharynx. The organic particles in the soil are utilized and the undigested matter along with the soil is passed out as small pills, called worm castings.

2. Burrowing
· Earthworm makes burrow by simply pushing the body through the soil if it is soft and by eating through the soil if it is hard.
· The burrows are internally cemented by the secretions of the cutaneous glands.
· The entrance of the burrow is often covered with faecal castings, pebbles or leaves to keep out water and enemies.
· The worm remains in the burrow with the anterior end facing upward and is fossorial (lives in self-made burrows).

3. Locomotion
· Earthworm moves by crawling or creeping, which is brought about by the longitudinal and circular muscles of the body.
· A wave of contraction, affecting the circular muscles, starts from the anterior end to the posterior one. Due to the contraction of the circular muscles of the anterior end, the latter becomes thin, elongated and extends forwards. At the same time, the setae of the anterior end hold the ground firmly and prevent the animal from slipping backwards. Now, the circular muscles of the anterior end relax and the longitudinal muscles contract. It causes the shortening and thickening of the anterior segments, and thus, the posterior part of the body is pulled ahead. The process is repeated and the worm is able to move forward with speed. Earthworms move at the rate of about 15 cm per minute.
· When the pattern of the waves of contraction is reversed i.e. from posterior to anterior, the earthworms crawl in the backward direction.

4. Breeding
· Breeding occurs in the rainy season.
· Earthworm is monoecious or bisexual or hermaphrodite viz. both male and female sex organs are found in one individual.
· Earthworm is protandrous i.e. testes mature earlier than the ovaries, so cross-fertilization occurs.
· Copulation involves a mutual exchange of worms.
· Earthworm is oviparous. Several eggs and spermatozoa are packed in the cocoon and on average four baby worms develop in one cocoon.

5. Regeneration
· Regeneration is well marked in earthworms. If cut into two parts, each part may develop the missing part and grow into a complete worm.
· A cut part of an earthworm can be grafted to another worm i.e. grafting is also possible in earthworms.

6. Defence
· Earthworm is a defenceless creature; however, it may eject the foul-smelling coelomic fluid through the dorsal pores when irritated.



External Morphology

1. Shape, Size and Colouration
· The cylindrically elongated bilaterally symmetrical body is pointed in front, thickest a little behind the anterior end and blunt behind.
· The size of the worm is varied; the mature worm is about 150-200 mm in length and 3-5 mm wide.
· The colour of the body is shiny dark brown due to the presence of ‘porphyrin’ pigment that helps to protect the body from ultraviolet radiations.
· The dorsal body surface is darker than the ventral surface and carries a dark coloured median longitudinal dorsal blood vessel which is seen through the integument.

2. Segmentation and Apertures in the body
· The body consists of 100-200 similar segments called metameres or somites.
· It exhibits metameric segmentation or metamerism or true segmentation i.e. the external segmentation corresponds to internal segmentation.
· Each segment is divided externally by groove and internally by circular septa.

External features of Earthworm


· The first segment is the peristomium and bears crescentic mouth. It is prolonged anteriorly into a fleshy lobe, prostomium which overhangs the mouth dorsally.
· Last segment or anal segment or pygidium (100 to 120th) bears vertical slit-like anus. It passes out indigestible food particles.
· Female genital aperture is a single median aperture situated on the mid-ventral surface of the 14th segment. The female reproductive bodies (ova) discharge through it during the breeding season.
· Male genital apertures are a pair of crescentic apertures situated in the ventrolateral position of the 18th segment. The male reproductive gametes or sperms are discharged through it during copulation.
· Genital papillae are two pairs of circular or rounded apertures found one pair in each 17th and 19th segments. They act as suckers during copulation.
· Grooves of segments 5/6, 6/7, 7/8 and 8/9 respectively bear four pairs of spermathecal pores. Each opening leads into a spermatheca in which the sperm of other earthworms, received during copulation, are stored.
· Intersegmental groove (except last) behind 12th segment bears dorsal pores (first one is situated in between 12th and 13th groove), from which coelomic fluid exits [BPKIHS 2005].
· All segments of the body (except 1 and 2) bear scattered openings of integumentary nephridia. About 200-250 such pores are scattered in each segment and 2000-2500 in the clitellar region (also called the forest of nephridia).
· Ventral surface of segments 17 and 19 bears genital papilla that helps in copulation.

3. Clitellum or cingulum
· It is a prominent circular band of glandular skin around the segments 14th, 15th and 16th. It secretes mucus, albumen, which helps in the formation of cocoon [MOE 2007], and is used for the fertilization of the eggs.

Clitellum of Pheretima


· Clitellar regions: On the basis of clitellum, the body of earthworm is divided into pre-clitellar, clitellar and post-clitellar regions.

4. Setae
· These are present in all the segments except the first, last and clitellar region [MOE 2002].
· About 80-120 setae are present in each segment. They are arranged in annular rows in the mid-ventral surface of each segment and such arrangement is called perichaetine arrangement. Each seta is a minute, elongated, s-shaped, faint yellow in colour, with swollen structure at the middle called nodulus.

Setae of Earthworm


· Each seta (singular of setae) lies in a setigerous sac, which is a small pit in the skin.
· Setae help in locomotion, climbing out of the burrow and in copulation by keeping the two earthworms together.

5. Body Wall
· The body wall consists of the cuticle, epidermis, muscular layer and coelomic epithelium (parietal peritoneum).
· Cuticle is thin, non-cellular, elastic secreted by the epidermis.
· Epidermis is thick-layered and lies just below the cuticle and consists of supporting cells, glandular cells consisting of mucus or goblet cells and albumen or cocoon forming cells, basal cells (lying between gland cells and supporting cells), receptor cell or sensory cells and setigerous (seta forming) cells.
· Muscular layer lies below the epidermis and consists of circular muscles, longitudinal muscles, protractor muscles and retractor muscles.
· Coelomic epithelium or parietal peritoneum is the innermost layer of the body wall.

Functions of the body wall
a. It provides definite shape and protects from physical and chemical injury.
b. The mucus secreted by mucus gland cells keeps the body moist, protects the body from desiccation, kills harmful bacteria and helps in locomotion.
c. The moist body wall helps in respiration.
d. Alternate contraction and relaxation of circular and longitudinal muscles perform locomotion.
e. Receptor cells are sensory in function and respond to touch, chemical and temperature.
f. Albumen cells secrete albumen which provides nutrition to the developing embryo.

6. Septum
· In Pheretima posthuma, septa are present in all the segments except the first four segments and 9/10. [IOM 2006]
· First septum lying between 4/5 is thin, membranous and oblique.
· The next four septa are thick, muscular and obliquely placed between 5/6, 6/7, 7/8 and 8/9.
· The first nine septa (4/5, 5/6, 6/7, 7/8, 8/9, 10/11, 11/12, 12/13 and 13/14) are complete (without any apertures) and remaining septa (14/15, 15/16, 16/17, 17/18 and so on up to the posterior end) are incomplete (with perforations).

7. Coelom (Body Cavity)
· Body cavity of the earthworm is a true coelom (Schizocoel) lined by coelomic epithelium formed by splitting of embryonic mesoderm.
· The coelom is filled with alkaline, milky white fluid called coelomic fluid. It consists of water, salts, proteins and coelomic corpuscles. Coelomic corpuscles are of the following four different types.
· Phagocytes or amebocytes or eleocytes are numerous, largest, nucleated and amoeboid corpuscles and contain a large number of ingested granules. They help to kill the foreign microbes present in the coelomic fluid.
· Mucocytes or granulocytes are elongated cells each having a broad fan-like process at one end and narrow nucleated stalk-like part at the other ends.
· Circular nucleated cells or leucocytes are rounded nucleated cells with clear cytoplasm.
· Chloragogen cells or yellow cells are small star-shaped cells. These cells are excretory in function. They regulate the level of fatty acids. It is considered that they are analogous to the vertebrate hepatic cells [BPKIHS 2007; IE 1999].

Functions of coelomic fluid
a. It serves as a hydraulic or hydrostatic skeleton [MOE 2065] to assist the musculature during locomotion.
b. The coelomic fluid keeps the internal organs moist, also keeps skin moist and helps in respiration.
c. Phagocytes help in defence.
d. It forms the protective covering around the internal organs of the body.
e. Some excretory matters also get rid of through coelomic fluid.
f. It facilitates the transportation of digested food from one part to another part of the body.


Interaction with Mankind

· Earthworms are known as “friends of farmers” are of much importance to mankind:
· They serve as nature’s ‘little tractor’ as they bring up the soil from below to the top. The worm casting is rich in urea and ammonia and improves the fertility of the soil.
· They are used as the bait for fishing.
· The turning over of the soil sometimes brings the pathogenic organisms inhabiting dead animals and plants on the surface.
· This is potentially risky as this may spread viruses, bacteria, nematodes injurious to man and economically important animals.
· Earthworms are of medicinal use in the Ayurvedic and Unani medicine system.
· Their body extracts are used as drugs in the treatment of renal stones, pyrrhoea, gout, impotency, jaundice etc.



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