Grade 11 Zoology Note

Detailed Study of Earthworm

Phylum:  Annelida

Class:  Oligocheata

Genus:  Pheretima

Species: posthuma

Common Name: earthworm

Habit and Habitat:

Earthworm is a burrowing animal usually live inside the soil surface in solitary. It burrows by ingesting soil from the mouth and liberate out from the anus on the soil surface as castings.

 # It feeds upon the dry or green leaves, dead materials, insect larvae, eggs, etc.

# Usually earthworm is widely distributed in fertile or loam soil which burrow at the depth of about 6″ inches.

# During dry season. It can grow at the depth pf 1m but during rainy season, found on the soil surface.

# Earthworm is commonly called friend of farmer or natural plough.

# the distribution of earthworm is almost cosmopolitan found from the sea level up to the altitude of 3000m.


1. Long, cylindrical, almost pointed at the both ends, metamerically segmented.

2. Number of segment varies between 100-120, each segment is also called somite or metamere and divisible into dorsal, ventral, anterior and posterior regions.

3. Dorsal portion is dark in colour due to deposition of skin pigment called Porphyrin.

4. Anterior portion is very close to clittellum and posterior region is far from the clittellum.



It includes Digestive tract or Alimentary canal or Gut.

Fig: Digestive system



It completes from anterior mouth or prostomium up to posterior anus includes buccal cavity pharynx, oesophagus, stomach and intestine.

The mouth is highly elastic can protrude out and protrude in surrounded by 1st body segment acting as lips called peristomium. Mouth follows short buccal cavity lies within 2nd and 3rd body segments.Buccal cavity is divisible into upper semi circular dorsal chamber. When this chamber gets extended size of buccal cavity becomes larger.

Buccal cavity follows another short phyarynx lies in the 4th,5th, and 6th segments of the body marked by pharyngeal mass on the dorsal pharynx. This mass secrete saliva or mucin and proteolytic enzyme. Due to heavy pharyngeal mass, pharynx is also depressed into upper, dorsal and lower ventral chambers separated by horizontal shelve. The wall of the dorsal chamber has ciliated epithillium cells which produces ciliatary current help in the collecton of enzyme. The collected saliva and enzyme 1st diffuse into dorsal chamber. So it is also called salivary chamber.

Pharynx follows short oesophagus in 7th, 8th segment of the body. In the 8th or 8/9th segment modifies into hard and oval gizzard surrounded by numerous circular muscles. The internal wall is also covered by hard cuticle layer. The contraction and relaxation of circular muscles easily masticate food and soil. Thus acting as grinding organ.

Gizzard follows short simple tube stomach extending from 9th upto 14th body segments. The internal wall of the stomach also secrete proteolytic enzyme.

 Stomach follows the longest portion of the alimentary canal intestine extending from 15th to last but one segment of the body. It has large diameter and looks brownish in colouration.

Structurally and functionally intestine is divisible into three regions;

1. Pre-typhlosolar region or digestive region

2. Typhlosolar region or Absorption region

3. Post- typhlosolar region or solar region

At last post-typhlosolar region opens to posterior anus through which release out food waste and soil particles in the form of castings.


PHYSIOLOGY OF DIGESTION: The physiology of digestion includes ingestion, digestion,absorption and egestion. If the food is green, first of all take into their burrow then mix with mucins secreted by the skin which makes colourless food after that including other foods with soil ingest through mouth follows buccal cavity and pharynx.

Gizzard: In gizzard, completely powdered making more easy. The main significant of ingesting soil is only to produce more friction during breaking down of food.

Stomach:In stomach ones more protein is partially digest as in the pharynx.In the pre-typhlosolar region, food is completely digested as follows;Enzymes from the wall of the intestine.

a) Enzyme trypase – react upon protein which convert into peptones and finally digest into amino acid.

b) Enzyme lipase – react upon fat acid which convert into fatty acid and glycerol.

c) Enzyme amylase – react upon carbohydrate convert into sugar and finally into glucose or fuctose.

After almost digestion, digested foods including undigested foods soil posses over to tryphlosolar region in which all the digested food molecules are absorbed by typhlosole and circulate through blood vascular system.Remaining portion of food and soil passes over to post typhlosolar region for storage. At last such substances eject out from the anus in the form of castings.


Nephridia: Nephridia are the excretory organ of earthworm which are very minute, coiled, eliminate waste either outside the skin or into the alimentary canal.On the basis of structure and position, nephridia is of following types:-

1. Integumentary Nephridia – in the skin

2. Pharyngeal Nephridia- only in the pharynx

3. Septal or Intentine or typical Nephridia – only in the intestine


On the basis of elimination

1. Exo-nephric nephridia e.g integumentary nephridia

2. Endo-nephric nephridia e.g pharyngeal + septal


In the pharynx good mix with saliva and treat by proteolytic enzyme which partially digest protein into peptones or peptides, therefore process of digestion start from the pharynx.

1. Integumentary nephridia:

The nephridia are very minute, coiled attached directly in the skin distributed from second segment upto last but one segment. Each segment has the numbers 200-250 but in the clitellum region has 2000-2500 numbers. Single nephridium does not possess mouth or nephrostone, eliminate nitrogen waste directly outside the skin. Thus, it is exo-nephric nephridium.


2. Pharyngeal nephridia:

They are only found in the pharynx i.e. in 4th, 5th and 6th segments.

 Each segment hasone pair of cluster of nephridium whose eliminating ducts opens directly in the buccal cavity and pharynx. 6th pharyngeal nephridia eliminate in 2nd segment( buccal cavity). 5th pharyngeal nephridia eliminate in 3rd segment(buccal cavity). 4th pharyngeal nephridia eliminate in 4th segment (pharynx). Hence it is a type of endo- nephridic nephridia.


3. Septal nephridia:

They are commonly called intestinal nephridia because only found in the intestinal region. It is also called as typical nephridia because size of nephridia is larger and eliminate maximum amount of nitrogen waste incompare to integumentary and pharyngeal nephridia. All the necessary regions are also present in this nephridium.

In each segment, number of nephridia varies between 80-100 i.e each septum possess 40-50 in number. Each septal nephridium consists of mouth and main body. Mouth is ciliated produces ciliary current which help in the selection of nitrogen waste in the body coelomic fluid. Mouth or nephrostom connected to terminal duct due to short neck.

Main body comprises of almost flat colourless straight lobe follows twisted loops which number varies between 6-10 always last loop is called terminal loop. Each twisted loop is made of distal loop, continuation of terminal duct.

Terminal ducts of septal nephridia opens separately into septal excretory ducts, again septal excretory ducts of each segment finally opens to supra intestinal excretory ducts. From these duct, waste diffuses into the intestine which eliminate with castings. Hence septal nephridia are also the type of endo nephric nephridium.

                     Fig: Different types of Nephridia.


 Earthworm is a hermaphrodite animal because male and female reproductive organs are both found in the single animal even then there is no self fertilization always cross fertilization because of protandrous character.


Male reproductive organ:

1. Two pairs of testes lies in the 10th and 11th segment.

2. Two pairs of seminal vesicles lies in the 11th and 12 th segment.

3. Two pairs of vasadeferentia ducts 12th segment to 18th segment.

4. Two testis sacs – 10th and 11th segments.

5. Two prostate glands extend from 17th to 21st or 22nd.

6. Two common ducts or copulatory organics pseudopenis in 18th segments.

7. Two male genital opening in 18th segments.

8. Two pairs of accessory glands each pair in 17th and 19th segments.

 Female reproductive organ :

1. One pair of ovary lies in 13th segments.

2. Two oviducts in 13/14 segments

3. Single female genital opening in 14th segment.

4. Four pairs of spermathecae, each pair in 6th, 7th, 8th and 9th segment.

5. Two pairs of spermaducal funnels in 11th and 12th segments.

Male reproductive organ:

Testes each pair lies in the 10th and 11th segments. Manufacture spermatozoa by the process of spermatogenesis. Spermatozoa swims actively in the testis fluid present in the testis sac of 10th and 11th segments. The 10thtestis deposits its spermatozoa in 11th seminal vesicles. 11thtestes deposits its spermatozoa in 12th seminal vesicles. These seminal vesicles provide nourishment till maturity. After the maturation, a spermatozoon again swims back in the testis fluid. Later collect by two pairs of spermaducal funnel of 10th and 11th segments. Spermaducal funnels follows 2 pairs of vasa deferentia duts extending from 12th to 18th segments. These ducts carry away and restore spermatozoa in the respective prostate glands.

There are two prostate glands looks colourless, leaf life, almost flat extending from 17th to 20th or 21st segment. Prostate gland provides nourishment till copulation. Vasa deferentia ducts and internal prostate ducts or copulatory organ or pseudo penis which opens through male opening of the 18th segment.

Beside these male organs also possess two pairs of accessory glands. Each pair is 17th and 19th segment serve as adhesive organs because their secretion help during copulation.

 Fig: Male reproductive organ.

Female reproductive organ:

Primary sex ovary is locate in 13th septum possess finger like projections containing different stages of eggs. Eggs within the ovary develop by oogenesis process. At maturation wall of the ovary rupture release eggs in the body cavity which are collect by 1 pair of oviducal funnel of 13th segment. It follows oviduct in 13th, 14th segments and opens commonly in the 14th segments as female genital opening.

There are four pairs of spermathecae, each pair lies in the 6th, 7th, 8th and 9th segments. Each spermathecae has larger sac ampulla for nourishment and small sac diverticulum which restore sperms collected at the time of copulation. Spermatica although related with the collection of sperm even then kept in the female reproductive organs because of receiver bag.

Fig: Praithima, ovary


During breeding season or rainy season at night earthworms come out from their burrows on the surface, they select their partners by themselves and temporary fuses in ventral to ventral position in head to tail arrangement. After the fusion, common ducts of one penetrate into the one pair of spermathecal of the other. After that deposit sperm in the diverticulum. Hence the main significance of copulation is to exchange sperm in between the two animals. Ater completion, copulation ended i.e separated. For the temporary fusion, accessory glands secrete adhesive fluid.


Cocoon formation: After copulation, earthworm moves into their own burrows. In the burrow, glanular cells of the clitellum become very active whose secretion form a tight sac around the clitellum called clitellar sac. After that earthworm’s body withdraws backward. As a result, sac collects eggs from 14th opening and sperms from spermathecal openings, at last comes out of the anterior region in barrel shape called cocoon.


       Fig: Process of Cocoon formation


Fertilization: Fertilization is external although takes place inside the cocoon which converts into zygote. After few hours, zygote undergoes cleavage give directly young ones. Therefore, the development is direct.



Fig: T.s nerve cord of Pherithima

Nervous system is annulated type, constitute central nervous system, peripheral nervous system and autonomic nervous system.

1.Central Nervous System: Central nervous system related with cerebral ganglia or brain in the 3rd segment and ventral nerve cord extending from 5th segment to 2nd last. Minute colorless brain locate just above the buccal cavity in 3rd segment connect with the ventral nerve code by circum pharyngeal connectives. Thus form a ring called nerve ring or circum pharyngeal ring. At the lateral sides of the ring in 4th segment present sub pharyngeal  ganglia and from 5th segment to 2nd last present segmental ganglia.

2. Peripheral Nervous System:It is related with nerve fibrous of the ganglia.It includes nerve fibers or nerve, which arises from central nervous system.

- From cerebral ganglia, 8-10 nerves arise and supply to prostomium, buccal chamber, and pharynx.

- From circumpharyngeal connectives, two pairs of nerves arise and supply to 1st and 2nd segment.

- From subpharyangeal ganglia, three pairs of nerve arise and supply to 2nd, 3rd and 4th segment.

- From each segmental ganglion, three pairs of nerves arise and supply to respective segment.


3.Autonomic Nervous System: It is primitive form because of lower invertebrate animal.


1. Reception of stimuli.

2. Conduction of stimuli.

3. Conversion of stimuli into messages.

4. Conduction of messages towards affective region.


Economic importance

Beneficial aspects

- It makes the burrow in soil. This is useful for plants and they can grow easily.

- They can be used as food.

-They are used as medicines to cure stone in bladder, jaundice etc.

- They can be used as baits for fishing.

- There are used in laboratory for research.


Harmful aspects

- They can damage plants in garden.

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