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Earthworm
Kingdom
: Animalia
Phylum
:
Annelida
Class
:
Oligochaeta
Genus
: Pheretima
Species
: posthuma
Common Name
: Earthworm
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Habit and Habitat:
Pheretima is commonly found in South East Asia, Japan, Sri Lanka, Australia
etc. It is a nocturnal terrestrial invertebrate, living in damp, moist, humus
rich soil. It prefers to live in burrows (12-18 inches) during daytime and at
night and during the rainy season they come out of their shelter. It feeds upon
the soil, leaves, dead materials, insect larvae, eggs, etc.
The soil, upon which it feeds, passes through
the body and is deposited as small rounded pellets or balls known as castings.
While burrowing, the earthworm makes the soil loose and porous and increases
fertility of soil.
External features
The body of earthworm is
long, narrow and cylindrical measuring about 15-20 cm in length and 3-5 mm in
thickness. The anterior end is more pointed than the posterior end. The dorsal
side of the body is dark brown in color (due to the pigment porphyrin which
protects the body from bright and strong light) and ventral side is lighter in
color.
The body is made up of
100-140 distinct segments called metameres separated from each other by
inter-segmental grooves or annuli (metamerically segmented). The first body
segment is called peristomium which bears the mouth on the ventral surface. The
peristomium anteriorly bears a small, fleshy lobe-like structure called prostomium.
A glandular band situated in 14th to 16th segments from the anterior end of an
earthworm is called Clitellum or cingulum. It forms a cocoon during the
reproduction.
Setae
The locomotory organs of
earthworm are the setae. Seta is an elongated, ‘s’-shaped structure, composed
of chitin, embedded in an epidermal pit called a setigerous sac or setal sac.
It is about 0-2.4 mm in length and 0-0.3 mm in breadth.
The distal end of the
seta projects above the surface of the skin. The proximal end of the seta lies
embedded in the skin, called the base and the middle region is swollen and
called the nodulus. Setae are present in all segments (about 80-120 in number)
except in the first, last and the clitellar segments and help in locomotion.
External apertures
(i) Mouth
(peristomium):
It is a crescent-shaped
aperture situated ventrally below the prostomium. It helps in taking in food
particles.
(ii) Anus:
It is a round aperture
situated at the posterior end of Earthworm in the last segment (anal segment).
It is the outer opening of the alimentary canal by which undigested food and
mud are excreted.
(iii) Female Genital
Aperture:
The female genital
aperture is a single opening situated on the ventral surface of the 14th
segment. Ova are discharged through these pores.
(iv) Male Genital
Apertures:
These are paired
openings situated on the ventrolateral sides of the 18th segment. Sperms are
released through these pores.
(v) Genital papillae:
A pair of genital
papillae are situated on the 17th and 19th segments each (above and below the
male genital aperture). They help in copulation.
(vi) Spermathecal pores:
4 pairs of spermathecal
pores are situated on the ventro lateral side of the intersegmental grooves of
segments 5/6, 6/7, 7/8, 8/9. They receive and store sperm during copulation.
(vii) Nephridiopores:
These are minute
openings of the integumentary nephridia. They are present in all segments
(about 200-250 in each segment, 2000-2500 in clitellar segments) excepting the
first six and the last one. They help to remove metabolic wastes of the body.
(viii) Dorsal Pores:
These are the mid-dorsal
openings on each inter-segmental groove behind the 12th segment. The coelomic
fluid comes out to increase the surface moisture through these pores.
Body Wall of Pheretima:
The body wall of
earthworm is thin, soft and slimy. It is composed of thin cuticle, an
epidermis, a musculature and coelomic epithelium.
a. Cuticle
It is a thin, elastic
and non-cellular protective outer layer. It is made up of parallel collagen
fibers and is perforated by numerous pores of epidermal glands.
b. Epidermis
It is a single layer
present below the cuticle. It is made up of four types of cells:
(i) Supporting Cells: These are tall and columnar cells, forming a
major part of epidermis.
(ii) Gland Cells or
Goblet Cells: These are secreting
cells present between supporting cells which keep the skin slimy and moist.
Gland cells may be club shaped mucus cells that secrete mucus or cylindrical
albumen cells that secrete albumin.
(iii) Basal Cells: They are small rounded or conical cells lying
between gland cells and supporting cells. They can replace other types of
cells.
(iv) Sensory Receptor
Cells: They are columnar cells
arranged in groups. They contain minute hair-like processes which help to receive
stimuli.
c) Muscular layer:
Below the epidermis
there are 2 different antagonistic muscle layers. Outer thin Circular muscle
forming a continuous layer around the body and inner thicker longitudinal
muscle layer, running in parallel bundles, separated one another by connective
tissue.
d) Coelomic epithelium
or peritoneum:
It consists of a thin
membrane of a single layer of flattened squamous cells. It is the outer wall of
the coelomic cavity. So it is also called the somatic or parietal layer of
Coelomic epithelium or peritoneum.
Coelom:
Coelom is the fluid
filled body cavity lying between the body wall and the alimentary canal.
The coelom in earthworm
is a true coelom (schizocoel) formed by splitting mesoderm. The coelom opens to
the exterior by dorsal pores and nephridiopores.
The coelomic cavity is
filled with an alkaline milky-white coelomic fluid that consists of plasma and
different types of nucleated corpuscles.
The four types of
corpuscles are:
(i) Phagocytes
(granulocytes):
These cells are
numerous, granular, large sized and amoeboid corpuscles. They contain many
ingested granules of bacteria. They help in removing harmful bacteria, and
other microorganisms.
(ii) Circular Nucleated
Cells (leucocytes):
These are circular,
nucleated, small sized, non granular cells with clear cytoplasm. These are
fewer in number (about 10% )
(iii) Mucocytes:
These are elongated,
vase-like cells having a broad, fan-like process attached to a narrow nucleated
body.
(iv) Chloragogen Cells
(yellow cells):
These cells are
numerous, star shaped, small sized cells having yellow strain. They are
supposed to be excretory in function.
Function of coelomic
fluid
·
Distribution of digested food materials.
·
Coelomic fluid comes out and keeps the skin moist.
·
Removes excretory wastes.
·
Forms the protective covering around the visceral organs.
·
Help in fixing the setae in ground.
·
Acts as a hydraulic skeleton and makes the body rigid.
Digestive System of Pheretima:
Digestive system is a
group of organs which helps in digestion where large food particles break down
into simpler forms and are absorbed to produce energy. It consists of an
alimentary canal and associated digestive glands.
Alimentary Canal
Alimentary canal is a long and straight tube
of varying diameter, which runs through the entire length of the body from
mouth to anus. It is held in position by the intersegmental septa. It consists
of following parts:
Mouth
: 1st segment
Buccal Cavity : 2nd-3rd or middle of 3rd segment
Pharynx
: 3rd-4th segment
Esophagus : 5th-7th segment
Gizzard
: 8thor 8/9segment
Stomach
: 9th -14thsegment
Intestine
: 15th up to anus
Anus
:
last segment
Mouth
It is a crescent aperture situated in the 1st segment below the prostomium.
Buccal Cavity
It is a short, thin-walled tube which
extends up to the middle of the 3rd segment. Earthworm
protrudes and retracts its buccal chamber, which acts as an organ of ingestion.
Buccal cavity leads to the pharynx.
Pharynx
It is a wide, thick-walled, pear-shaped
chamber, which extends up to 4thsegment. It is
separated from the buccal cavity by a dorsal transverse groove, lodging the
cerebral ganglia (nerve ring). The lateral walls of the pharynx are pushed
inside forming a narrow horizontal shelf on each side, dividing the pharyngeal
cavity into a dorsal salivary chamber and a ventral conducting chamber. The
pharyngeal mass produces saliva secretion from chromophil cells or gland cells
containing mucin & proteolytic enzymes (protease) which are poured into the
salivary chamber.
Esophagus
It is a straight, narrow, elongated & thin
walled tube extending from the fifth to seventh segment. It has no glands. It
passes the food particles from pharynx to gizzard.
Gizzard
It is an oval, thick-walled and highly
muscular organ lying in the 8th or 8th-9th segment. It is lined by a tough cuticle which
helps to chew the food. Its gut wall consists of only circular muscle.
Stomach
The part of the
alimentary canal lying between segments 9 to 14 is called the stomach. The wall
of the stomach is highly glandular, vascular and thrown into internal folds.
Both the ends of the stomach are provided with sphincter. The glandular cells
secrete proteolytic enzymes.
Intestine
It is a long, wide and thin walled tube,
extending from the 15th to the last segment, up to the anus. Its inner
lining is folded to form villi which are ciliated, vascular and glandular.
One of the villi becomes larger and well
developed than the other called typhlosole which extends mid-dorsally from 27th
segment backwards except last 25th segment which divides
intestine into 3 parts.
i. Pre-typhlosolar region
It extends from the 15th segment to the 26th segment. It consists
of villi but no typhlosole. In the 26th segment, there is a
pair of short and conical lateral outgrowth called intestinal caeca which
extends upward up to the 22nd segment and produce amylase for starch
(carbohydrate) digestion.
ii. Typhlosolar region
It is the 2nd part of the intestine which extends from the 27th segment to 25 segments in front of anus. It has both villi and
typhlosole. The typhlosole increases the absorptive surface area of the
intestine.
iii. Post-typhlosolar region
It is the last part of the intestine lying in
the last 23-25 segment in front of anus. It lacks villi and typhlosole. It is
also called rectum. It opens outside through the anus.
Anus
It is a circular opening in the last segment.
Undigested food materials release in the form of worm casting.
Histology of the
alimentary canal:
Peritoneum
It is the outermost
layer of the gut wall made up of tall and narrow cells. In the stomach and
intestine, this layer consists of chloragogen cells, containing yellow pigment
which are believed to be excretory in function.
Muscle layer
The peritoneal
epithelium is followed by longitudinal muscle fibers and circular muscle
fibers. They are well developed in the buccal cavity, Pharynx and esophagus. In
Gizzard, only a thick layer of circular muscle is found.
Enteric epithelium
The internal columnar
epithelium is formed of ciliated cells on the pharynx, glandular in the stomach
and intestine. It is folded internally in the esophagus, stomach and intestine.
Cuticle
A cuticle is present
only in the buccal cavity (thin layer) and in the gizzard (thick layer).
Physiology of
digestion
Food and Feeding:
Pheretima ingest soil,
rich in organic particles, seeds, decaying leaves, grasses, larvae, nematodes
etc. The buccal chamber is enlarged and the food is drawn into the mouth due to
contraction and relaxation of muscular pharynx.
Digestion:
Digestion is extracellular. The passage of
food in the alimentary canal is caused by peristaltic movement of the gut wall,
caused by the circular and longitudinal muscles.
In the pharynx, the digestive juice,
containing mucin & a proteolytic enzyme, secreted by the glandular cells of
the pharyngeal mass, mixes with the food. Mucin lubricates the food while
protease digests protein.
Thereafter the food passes down the esophagus
into the gizzard where it is ground up in a fine state. In the stomach the food
receives the proteolytic enzymes and calcium secreted by calciferous glands.
Calcium neutralizes the acidity of soil.
In the intestine the food is mixed with
pepsin, trypsin, amylase cellulase, lipase etc. which helps in the digestion of
proteins, starch, cellulose, fats, oils etc.
Absorption:
The digested food is absorbed by the
typhlosole intestine and passed into the bloodstream foe distribution to the
various parts of the body.
Egestion:
Undigested food and soil are consolidated
inside the rectum and egested through anus in the form of worm castings.
Excretory system of Earthworm
The process of removal of metabolic waste
products including nitrogenous material like ammonia, urea, uric acid, amino
acid, etc. from the body is called excretion. Excretion in earthworm is
performed by long, thin and coiled tubules called Nephridia. They are found in
all segments except the first two.
Types of Nephridia
According to their position in the body, they
are of 3 types:
Septal Nephridia- found on both side of septa behind 15th segment
Pharyngeal Nephridia- Found on 4th, 5th and 6th segment
Integumentary Nephridia- Found in the body wall
Septal nephridia
They are well developed and the largest
nephridia, and found attached to both sides of each intersegmental septum
behind the 15th segment. Each septum has 80-100 septal nephridia arranged in
two rows.
Each septal nephridium consists of the
nephrostome, neck, body of nephridium and the terminal duct.
a. Nephridiostome (Nephrostome)
It is a proximal, wide, circular, funnel
shaped structure present in the Coelom. It consists of a mouth-like opening,
which is surrounded by two ciliated lips, larger upper lip and smaller lower
lip. It opens into coelom from where it collects excretory products.
b. Neck
Funnel leads into a short and narrow ciliated
neck, which joins the nephrostome with the main body of the nephridium.
c. Body
It is the main tubular part of the nephridia
which is coiled around its axis.
It consists of two parts, a short
straight lobe and a long twisted loop. The twisted loop consists of proximal
and distal limb, which are spirally twisted upon each other. Proximal limb is
attached to the neck and terminal duct of the twisted loop, while the distal
limb to the straight lobe.
The number of twists varies from 9-13.
d. Terminal duct
Proximal limb of the body of nephridium ends
in a short and narrow duct called the terminal duct. All the terminal ducts of
a segment open into the septal excretory duct of their side. Septal excretory
ducts collect the excretory products and transfer them to a pair of
supra-intestinal excretory ducts.
Function of septal nephridia
Septal nephridia discharges the waste products
into the lumen of the intestine. So they are called enteronephric nephridia.
2. Pharyngeal nephridia
They are found as paired tufts on either side
of the 4th, 5th and 6th segment. They are similar to septal nephridia but they lack
nephrostomes. The terminal ducts of all nephridia of each segment
unite to form a single common pharyngeal ducts in each side.The duct arise from
6th segment opens into buccal cavity in 2nd segment, the ducts of 4th and 5th segment opens into the pharynx in 4th segment.
Function:
They discharge the waste products directly
into the buccal cavity and pharynx from where these are passed outside with
undigested food through the anus. So they are called enteronephric nephridia.
3. Integumentary Nephridia
They are attached to the inner side of the
body wall from 2nd to last segment. They are smaller than septal
nephridia. They also lack nephrostomes or funnels.
Their terminal ducts open on the outer surface
of the body wall by nephridiopores.
They are about 200-250 in each segment but
about 2000-2500 in clitellar region. So, clitellum is called the forest of
nephridia.
Function:
They discharge nitrogenous metabolic waste
products directly outside from the body through nephridiopores. So they are
called exonephric nephridia.
Physiology of Excretion
Earthworm is ureotelic because its nitrogenous
waste matter consists of 40% urea, 20% ammonia, 40% amino acid and other
nitrogenous compounds. The nephridia are richly supplied with capillaries; they
collect the nitrogenous waste and remove it from the body. Integumentary
nephridia discharges nitrogenous waste products directly outside the body
through nephridiopores. So they are called exonephric nephridia. Septal
nephridia and pharyngeal nephridia discharge the waste products into the lumen
of the alimentary canal eliminated with feces. So they are called endonephric
nephridia.
The beating of cilia of the nephridiostome
drives a flow of coelomic fluid with metabolic waste. When the excretory
matters move in the nephridia they are converted into urea and ammonia to be
excreted.
Nervous system of Earthworm
The system which controls and coordinates all
the body activities according to internal and external environment is called
the nervous system. The nervous system of earthworms is well developed. It
consists of 3 parts:
Central nervous System
Peripheral nervous system
Sympathetic nervous system/Autonomic
Central nervous system
This part of the nervous system lies along the
midline of the body.
It consists of 2 parts:
Nerve ring and Ventral nerve cord.
a. Nerve ring
It is a ring-like spherical structure which
lies around the pharynx in the 3rd and 4th segment.
It has 3 parts:
i. Supra-pharyngeal ganglia (cerebral
ganglia)
A pair of whitish pear-shaped supra pharyngeal
ganglia fused to form the brain lies dorsally in the 3rd segment in the
depression between the buccal cavity and the pharynx.
ii.Circumpharyngeal connectives/
Peri-pharyngeal connectives
From either side of supra-pharyngeal ganglia,
a pair of thick short bands of circum-pharyngeal connectives arise which
enclose the pharynx.
It is outer bulging part which extends from
3rd to 4th segment
iii. Subpharyngeal ganglia
Ventrally Circum-pharyngeal ganglia meet with
a pair of subpharyngeal ganglia. It is the lower region of the nerve ring which
lies in 4thsegment.
b.Ventral nerve cord
It is a long thread-like double layered
structure, which arises from the posterior part of sub-pharyngeal ganglia and
runs posteriorly up to the last segment of the body. It extends from the 5th to
last segment. It has a swelling bulb-like structure in each segment called
segmental ganglion.
NOTE: Nerve cords are two in number but they
are fused. So they are called double ventral nerve cords.
Peripheral nervous system
Those nerves which arise from different parts
of the central nervous system are called peripheral nervous systems.
Following pairs of nerve arise from brain:
8 to 10 pairs of nerves arise from the
brain/ supra pharyngeal ganglia which communicate or innervate or
supply to the prostate, buccal cavity and pharynx.
2 pairs of nerves arise from cirum-pharyngeal
connectives which supply to the wall of prostomium and peristomium
segments.
3 pairs of nerves arise from subpharyngeal
ganglia which supply to the 2nd, 3rdand 4thsegment.
3 pairs of nerves arise from each segmental
ganglia which supply to the various parts of the segment, gut wall, body
wall and other internal organs.
Sympathetic nervous system
Sensory and motor nerve fuses to form nerve
plexuses which are situated in the muscle of the body wall and gut wall.
These plexuses are connected with the peri-pharyngeal connectives.
Physiology of nervous system
All the activities of earthworms are under the
control of the nervous system. The nerves consist of both sensory and motor
fibers. The afferent nerve fibers start from sensory cells or receptor organs
and terminate into ganglia of the central nervous system and form synaptic
connection with dendrites of efferent neurons. The sensory impulse conducts
from the receptor by afferent fibers to the ventral nerve cord. From the
central nervous system, the impulse reflected back as motor impulse along
efferent fibers to effector organs. The effector organ gives a suitable
response to the stimuli.
Reproductive
System of Earthworm
•Earthworms are hermaphrodites
(monoecious) but they cannot fertilize their own eggs because of different
position of male and female genital aperture as well as they are protrandous
(i.e. male sex matures earlier than female gametes). So, cross-fertilization
takes place.
Male reproductive organs
They consist of following parts:
•Testes
•Testis sac
•Seminal vesicles
•Spermiducal funnel/ spermatic funnel
•Vasa deferentia
•Prostate gland
•Accessory glands
•Male genital pore
•Testes
There are two pairs of small, white and lobed
testes, located in the 10th and 11th segment. They are attached to the
posterior surface of 9/10 and 10/11 intersegmental septa. Each testis consists
of 4-8 finger-like projections containing round cells called spermatogonia.
They are enclosed within the testis sac.
Function: They produce sperm.
•Testis sac
Testes are enclosed by wide, thin-walled,
fluid-filled sacs called testis sacs. There are two testis sacs present in the
10th and 11th segment. Testis sac communicates with a pair of seminal vesicles
of succeeding segments.
The testis sacs of the 11th segment are larger
as they enclosed the seminal vesicles of that segment.
•Seminal vesicles
There are two pairs of large, white, spherical
seminal vesicles found in 11thand 12thsegments. The Seminal vesicles of 11th
segment communicate with the testis sac of 10th segment while those of 12th
segment communicate with the testis sac of 11th segment. They develop as septal
outgrowth, and are called septal pouches.
Function: Maturation of sperm takes place in
seminal vesicles.
•Spermiducal funnel/ spermatic funnel
There are two pairs of ciliated spermiducal
funnel lying below each testis in the 10thand 11th segment and enclosed within
the same testis sac.
Function: Mature sperms pass through the
spermiducal funnel into vasa deferentia
•Vasa deferentia
They are two pairs elongated, narrow,
ciliated, thread like tubular structure which extends from 12thto
18thsegment.In 18th segment, they join together with a prostatic duct of the
same side.
Function: They collect sperm from spermatic
funnels and pass to the prostate gland.
•Prostate gland
There is a pair of large, white, flat, solid,
irregular glands, extending from 16thor 17th to 20th or 21st. Segment. From
each prostate gland, a short, thick curved prostatic duct emerges in the 18th
segment. The prostatic duct joins the two vasa deferentia of its own side to
form a common prostatic spermatic duct which opens separately through a male
genital aperture on the ventral side of the 18th segment.
Function: It produces prostatic fluid which is
alkaline in nature. It activates sperm and it keeps sperm motile.
•Accessory glands
These are two pairs of whitish, spherical
structures present one pair in each of the 17thand 19th segment.They open to
the exterior by genital papillae. Secretion of these gland helps in holding two
worms during copulation.
Function: They help in pseudocopulation.
•Male genital pore
It is found in one pair located in the 18th
segment.
Function: It helps to release the sperm during
copulation.
Female reproductive organs
The female reproductive system consists
of
1.Ovaries
2. Oviducal funnels
3. oviducts
4. spermatheca
5. Female genital pore
•Ovaries
A pair of small, whitish and lobulated
ovaries located in the 13th segment, on either side of the ventral nerve cord
attached to the posterior part of the septum of 12/13 segments. Each ovary has
several finger-like processes with various stages of ova (mature in the distal
part and immature in the proximal part).
Function: They form ova.
•Oviducal funnel
Below each ovary, there is a small ovarian
funnel with folded and ciliated margin, which leads to oviduct.
Function: It receives the ova and passes to
the oviduct.
•Oviduct
Each Oviducal funnel leads to short conical
oviduct in 13th and 14th segment.They run posteriorly and converge to meet in
the body wall below the nerve cord to form female genital aperture mid
ventrally in 14thsegment.
Function:They collect ova from ovary and pass
to female genital pore
•Spermatheca
These are 4 pairs, one in each of the 6th,
7th, 8th, 9th segment situated ventro-laterally. Each spermatheca has a broad
pear-shaped body, the ampulla and a short narrow neck with a small diverticulum
in the inner side. Spermatheca opens outside through spermathecal pores.
Spermathecae are also called seminal receptacles as they store spermatozoa from
another worm during copulation.
Function: They store sperm in diverticula
during copulation and the ampulla provides nourishment for the sperm.
•Female genital pore
It is a single unpaired small pore, lying in
the 14th segment.
Function: It helps to release ova.
Copulation
The process of transfer of sperms of one worm
to another worm for the cross-fertilization is called copulation. Copulation
takes place during the rainy season (from July to October at night or early in
the morning before sunrise). During copulation two earthworms come closer and
are attached ventrally in opposite directions in such a way that the male
genital pore of one worm lies against the spermathecal pore of another. Both
worms remain united together by the secretion of accessory glands and also by
mutual penetration of setae in each other’s body. Sperm and prostatic fluid of
each worm are deposited in spermatheca through spermathecal pore. Copulation
lasts for about an hour. Then the worms separate and later (after some day)
they lay their eggs in a cocoon.
Cocoon formation
Cocoon formation takes place after copulation,
when ovaries mature. The gland cells of clitellum become active and secrete
viscous and gelatinous substance which hardens on exposure to the air to form a
membranous elastic girdle called cocoon or egg capsule. Several eggs are laid
in the girdle from the female genital pore. Then the worm starts to wriggle
behind so that the girdle slips forward. As the girdle passes over spermathecal
pores, it receives sperm of another worm from the spermatheca. Finally, the
girdle is cast off from the anterior end and soon the two ends close to form a
cocoon or ootheca. Many cocoons may be formed after each copulation because all
spermatozoa in spermatheca do not pass out at once.
In earthworm, fertilization is external that
occurs inside the cocoon, where each ovum is fertilized by sperm. Only one
fertilized egg in each cocoon undergoes further development.
The development is direct i.e. no larval stage
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