Earthworm Grade-11 Zoology COMPLETE NOTES

Earthworm Kingdom               : Animalia Phylum                  :  Annelida Class                      :  Oligochaeta Genus                     :  Pheretima Species                   :  posthuma Common Name      : Earthworm

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                    : 1^st  segment

Buccal Cavity        : 2^nd-3^rd or middle of 3^rd segment

Pharynx                  : 3^rd-4^th segment

Esophagus              : 5^th-7^th segment

Gizzard                  : 8^thor 8/9segment

Stomach                 : 9^th -14^thsegment

Intestine                 : 15^th up to anus

Anus                     : last segment

Mouth

It is a crescent aperture situated in the 1^st segment below the prostomium.

Buccal Cavity

 It is a short, thin-walled tube which extends up to the middle of the 3^rd 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 4^thsegment. 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 8^th or 8^th-9^th 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 25^th segment which divides intestine into 3 parts.

i. Pre-typhlosolar region

It extends from the 15th segment to the 26^th segment. It consists of villi but no typhlosole. In the 26^th segment, there is a pair of short and conical lateral outgrowth called intestinal caeca which extends upward up to the 22^nd segment and produce amylase for starch (carbohydrate) digestion.

ii. Typhlosolar region

It is the 2^nd part of the intestine which extends from the 27^th 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 15^th 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 4^th, 5^th and 6^th 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 6^th segment opens into buccal cavity in 2^nd segment, the ducts of 4^th and 5^th segment opens into the pharynx in 4^th 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 2^nd 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