Earthworm: Morphology & Anatomy (Old NCERT) - NEET PDF Download

EARTHWORM (PHERETIMA)

Systematic Position :

Kingdom : Animalia

Branch : Eumetazoa

Division : Bilateria

Subdivision : Protostomia

Section : Eucoelomata

Phylum : Annelida

(1) True coelomic body cavity arising as a split in mesoderm (schizocoel) and lined by mesodermal coelomic epithelium (Peritoneum)
Phylum : Annelida :

(1)"Bilateral symmetry", "triploblastic".
- Metamerically segmented body
- Body cavity segmented like body wall.
- Excretory organ - "nephridia"
- Locomotion = "setae"
- Non chitinous body

Class : Oligochaeta

(1) Cephalisation - absent
(2) Head, eye, tentacles - absent
(3) Setae in body wall
(4) Clitellum = For "Cocoon formation permanent.
(5) Usually hermaphrodite and no free living larval stage
(6) Parapodia absent

Order : Terricola
(1) Clitellum multilayered (2) Terrestrial animals

Genus : Pheretima

COMMON SPECIES -
"P. posthuma" - north Indian earthworm
discoverer - K.N. Bahal
Pheretima - In Japan, Sri Lanka and Australia
Eutyphaeus - North Indian earthworm.
Mega scolex - longest 3 m in length - South Indian earthworm
Drawida - South Indian earthworm
Lumbricus - American and Europian earthworm.

Earthworm - Fossorial (lives in burrows)
- Burrows found in moist soil, with more humus
- Cutaneous respiration
- absent in dry soil
- maximum in garden soil, nocturnal animals.

External Morphology :-

- Body of Pheretima has "100-120" segments
- Segments are known as metameres.
- First segment - "Peristomium, peristomium bears a terminal slit like mouth.
- A dorsal fleshy lobe like process overhanging the mouth like a hood called prostomium.
- Prostomium helps in digging burrows
- Photoreceptor also found in it.
- Inter segmental groove present between every two segments.
- In each of the four intersegmental grooves between segments 5/6 , 6/7 , 7/8 and 8/9 there is a pair of ventro - lateral elliptical spermathecal pores.
- "14- 15- 16" segments have a pink coloured layer around them. This thick collar or girdle like thickning is known as "cingulum" or clitellum. This is made up of glandular cells.

- First segment of clitellum or 14th segment has a "female gonopore". or female genital pore is located at the mid-ventral line of this 14th segment.

- 18th segment - "Ventro-lateral"- 1 pair male gonopore.

- 17 and 19 segments - Pair of genital papilla. on ventro-lateral position.

- They helps in binding two Pheretima in vetro lateral condition at the time of coapulation.

 Setae

- Each segment of body wall except first, last and clitellum bears an equatorial annular row (=perichaetine arrangement) of about 80-120 minute S-shaped and yellowish setae = Chaetae.

- Setae are half embedded in body wall and half projected backwards upon body surface.

- They help in locomotion

- Young Pheretima - Setae present on clitellum also.

- Nephridiopores : They are 200 to 250 minute pores of integumentary nephridia scattered upon the surface of each segment except the first six. 

- upon clitellar segments their number is about 2000 to 2500
- Mid - dorsal pores in each inter segmental groove after twelfth segment.
- Dorsal pores are absent in last groove.
- Body-fluid - oozes out always from these pores.
- this fluid moistens and smoothens the skin for cutaneous respiration.
- When salt or alcohol is sprinkled on its body the fluid rapidly oozes out and causes its death.
- A small and vertical slit like aperture at the end of last anal segment (Pygidium) known as anus.

Body Wall :- 4 main layers

(a) Cuticle :- "Thin" & semi transparent cuticle composed of white coloured irregularly arranged collagen fibers "Non chitinous" protective covering, pores are present.
(b) Epidermis : Second layers - Made up of columnar epithelial cells - 4 type of cells found
 

(1) Supporting cells :- Narrow, ordinary, unspecialized epithelial cells
- Forms major part so they are main cells.

(2) Basal cell :- Shorter, conical cells, wedged in between and narrow basal parts of other cells.
- Present at the base
- Also known as replacement cell, as they replace the destroyed cells of epidermis.

(3) Sensory cells :-
- Narrow columnar cells occurring here and there.
- Each cell has small sensory hair at it's free end to receives stimuli

(4) Gland cells :- Thicker cell distributed here and there between the supporting cells. They are of two types:

(c) Muscle layers :-
Outer = Circular muscular layers
Inner = Longitudinal muscle layers

Circular muscular layer :- Continuous layer around the body
- Porphyrin pigment present

Brown colour is due to this pigment protects from ultra violet rays. This is obtained by the decomposition of chlorophyll.

Longitudinal muscle layers :- (discontinuous)

It is broken into several longitudinal strips or bands, separated from each other by thin connective tissue.

- They appear elliptical or club shaped in transverse section.

(d) Parietal coelomic epithelium (= peritoneum) :-
Inner most layers : found next to the longitudinal muscle layer.
Thin, membrane like, originated from mesodermal epithelium of flattened squamous cells.
- outer envelope of coelomic cavity and hence, called parietal or somatic layer of coelomic epithelium or peritoneum.

Function of body wall :-

(1) Provides fixed shape and size to the body
(2) provides protection, mucous prevents from harmful bacteria
(3) fluid moistens the body surface and facilitates in respiration.
(4) Setae and muscles of body wall helps in locomotion.

Body cavity is "True coelon"

Mesodermal in origin and lined by two layers of mesodermal layers

Outer layer (Parietal - Peritonium)

Inner layer (Visceral - Peritonium)

Space between two above layers is called body cavity.

Body cavity is segmented

Septa present between every two segments. They are known as inter segmental septa.

First inter segmental septa - between "4-5" segment

9th and 10th segment do not have this septa

Istseptais- thin and straight

Next 5 septa - Thick and funnel shaped

Rest of all the septa - Thin

- First 9 septa are - non porous All posterior septa - Porous

each septa has "136 - pores"

these pores are surrounded by Sphincter.

Body cavity contains coelomic fluid which is - milky and alkaline fluid.

- Matrix of watery plasma contain proteins, salts and minute nucleated cells or corpuscles. They are of 5 types

(1) Granulocytes or eleocytes or phagocytes : Oval shaped

Function :- Phagocytosis of bacteria. Granular cytoplasm present, which serves to store nutrients.

(2) Amoebocytes cells:- Irregular shaped and phagocytic to bacteria.

(3) Mucocytes = Spiral cells.

Function :- unknown

(4) leucocytes or circular cells :- Flattened and circular cells.

function : Uncertain

(5) "Yellow cell or chloragogen cell" :- Analogous to liver of higher animals. Shape : Star shaped.

- Liver like functions.

Found in the wall of alimentary canal and on maturity they fall in the body cavity and gets destroyed.

function : (1) Storage of food : (Glycogen + lipid). The yellow colour is due to stored lipids.
(2) "Deamination and urea formation"
(3) Glycogenesis
(4) Lipogenesis

When the cell matures it breaks from the visceral peritoneum and falls into the body cavity where it gets decomposed.

GENERAL ANATOMY AND PHYSIOLOGY

Locomotion in earthworm:

- Given by "Grey" and "Lismann"

- Lateral appendages absent

(1) Setae - fix the body to substratum

(2) Muscular layer of body wall.

C.M.L. = elongates body

L.M.L. ( Shortens the body to make thicker) contraction wave - like

- moves in opposite direction of wave

- In Pheretima contraction in 9 - segments takes place at one time.

(3) Body fluid - "Hydraulic - skeleton"

(4) Mucous = helps in locomotion on smooth surface. Speed = 25 cm/min.

Function of coelomic fluid :

(1) works as hydro static skeleton which facilitates locomotion.

(2) The fluid helps in respiration to exist dorsal pores.

(3) Chloragogen cells helps in storage of food and excretion too.

(4) Because of this fluid the body of earthworm is lusturous.

DIGESTIVE SYSTEM

Buccal cavity : In (1-2) segments.

(1) Because of radial muscle the buccal cavity evertible.

- Dorsal wall of pharynx is thick and is known as "pharyngeal bulb".
In this cavity are present two lateral septa. These then divide the cavity into two chamber "incompletely".

In Dorsal part - Dorsal Chamber - Salivary chamber.

In the dorsal part of salivary chamber is present saliva secreting cells known as "chromophil cells".

In Ventral part - "conducting chamber" - This is a passage for food - Pharynx located in 3-5 segment
- 6-7 has oesophagus.

Oesophagous opens into a thick walled "gizzard". This is located in 8th segment.

Stomach :- In "9th - (13-14)^th "segments

- Sphincters present on both ends

- Gland cell in wall secretes "proteolytic" (Juice/Enzyme).

- Found in other earthworms (not in Pheretima) "calciferous gland" in wall of stomach.

Calciferous gland - neutralise humic acid by secreting lime.

Intestine :- Longest and widest part of alimentary canal.

Extends behind from the beginning of 15th segment to the (last 20-25 segments). This is somewhat compressed by each inter-segmental septum.

- It has a beaded appearance

- Wall thin and highly vascular and glandular, ringlike. These rings are called villi.

- Severalvilli-like internal longitudinal fold in most of the part.

- A specially large and prominant fold, called typhlosole hangs internally into the intestinal lumen from its mid dorsal line. Beginning from the anterior end of 27th segment and ending in front of the last 24 or 25 segments.

- Due to typhlosole, the alimentary canal is distinguished into pre-typhlosolar, typhlosolar and post-typhlosolar regions.

- last 20-25 segment contains rectum second longest part of alimentary canal.

- Two "blind duct" present in 26th segment attached to intestine.

- They are known as intestinal caecae.

- This is situated upto 22nd segment

- secretes digestive enzyme

Typhlosole & villi - increases the absorption area in intestine.

FEEDING AND FOOD

"Omnivorous"

Food - Humus, dead insects, dead leaves and twigs.

In feeding - Buccal cavity helps the most, while pharynx help as a sucking organ.

Digestion :- Starts in pharynx

Wall - Saliva (Mucous, protein, digesting enzyme endopeptidase enzyme)

This enzymes acts on protein and helps in the digestion of large protein molecules into smaller ones.

Gizzard - Performs grinding

Stomach - digestion same as in pharynx

Intestine - Maximum digestion takes place here.

(1) Amylase - Starch → Maltose

(2) Maltase - Maltose → Glucose

(3) Amylopsin - Glycogen → Glucose

(4) Chitinase - Chitin → N.Acetyl glucosamine

(5) Cellulase - Cellulose → Glucose

(6) Sucrase - Sucrose → Glucose + Fructose or invertase

(7) Lipase - Fats→ fatty acid + glycerol

(8) Exopeptidase - Peptones and proteoses → Amino acid

Absorption :- takes place in intestine mainly by the help of typhlosole

- Absorption takes place after 26th segment

- Blood distributes the digested food

Defaecelion :- the wall of rectum absorbs water from the undigested substances

- Faeces - in form of dry pellets - "castings"

EXCRETORY SYSTEM

Excretory organ - "nephridia"
3 types of nephridia
Septal Nephridia - found on "inter segmental septa"
- Starting from 15-16 segment all posterior septa have nephridia
- arranged in two rows on each plane of septa. Each row has 20-25 nephridia".
- 40-50 nephridia on each face of septa.
- Septal nephridia on one septa (80-100)
- In 15th and last segment these are 40-50 septal nephridia

Structure of septal nephridia:-
(1) Nephrostome - A slit like hole.
(2) Neck :- After nephrostome this part is a straight tube like structure.
(3) Twisted loop :- made up of two limbs ie. ascending and descending limbs.  "No of twist - 9-13 twists"  Terminal end of distal-limb is in form of a straight lobe nephridia connected with inter-segmental septa with the help of straight lobe.
(4) Terminal duct :- last part of nephridia.
- one ciliated canal inside terminal duct.
- Nephridia absorb body fluid with the help of nephrostome.
In twisted loops - re-absorption process takes place, useful material absorbed and poured into the body cavity.
- all excretory substance gets concentrated and reaches to the "terminal-duct".
- All terminal ducts present at the one lateral side of septa fuses and forms a septal nephridial duct or septa excretory canal. It forms on the posterior part of septum.
- In 15th segment, septal nephridial duct is absent it first seen in 16^th segment.
- two large ducts are present at the dorsal side of alimentary canal. From 15th to last segment they are known as supra intestinal excretory duct.

S.N.D. - opens in S.I. excretory duct.
- Supra - intestinal excretory duct open through a small pore in each segment into alimentary canal.
- alimentary canal absorbs water from excretory materials.
- Excretory substances with undigested food thrown out from anus.
- Septal nephridia - "endonephric" or "entero nephric"
In order to check the loss of water excretory substances are thrown into alimentary canal. so septal nephridia help in "osmoregulation"

Terminal duct → S.N.D. → Supra Intestinal excretory duct → Alimentary canal

(2) Integumentary nephridia:-
- Present on the internal surface of body wall.
- After 6th segment - each segment - 200-250
In clitellar segments they are 10 times (2000-2500) They are known as "forest of nephridia"
- small in size so - they are known as "meronephridia"
- Holonephridia - septal nephridia are large in size and less in number
Nephrostome absent - called - "protonephridia"
If nephrostome present - meta nephridia.e.g. septal nephridia
Twisted loop `V' shaped
Their terminal duct - opens outside the body so are "exonephric".

(3) Pharyngeal nephridia :
Present on dorsal side of alimentary canal
- In 4-5-6- segment they are mixed with blood glands.
- In each group - 100 nephridia
- Total 600 nephridia size is same as septal nephridia.
Structure is same as integumentary nephridia.
Terminal ducts of each group unit to form a common duct.
Common ducts of 4-5th segment open in pharynx in 4 segment. While of 6th segment open in buccal cavity in 2nd segment.
- Called "enteronephric."
- They are not related with excretion, but perform the function of digestive glands.
Modern scientist - "peptonephridia"
Pheretima - "Ureotelic"
When moisture is more than "Ammonotelic"

REPRODUCTIVE SYSTEM
Earth worm is "monoecious" - hermaphrodite.
Reproductive organ - Present on ventral side of alimentary canal.

Male reproductive organ :-

- 10th & 11th segment has testes sac.
Testes sac. - formed by coelomic cavity each testes sac has a pair of testes.
Free end of testes is divided in 4 lobes, These lobes produce spermatogonia
Testes - mesodermal in origin.
Isttestes sac - smaller
IIndtestes sac- larger.
A pair of seminal vasicles associated with each testes sac.

1 pair - 11th segment - smaller seminal vesicle
I1 pair - 12th segment - large seminal vesicle

Each seminal vesicle is connected to testis sac by a pair of tubules.
- Spermatogenesis - in "Seminal Vesicle" testes forms only spermatogonia
- Monocystis protozoa, found in testes or seminal vesicle. they destroy the sperm - "male sterility
- From each testes sac arises a pair of vas deferentia.

They extends upto 18th segment
- A pair of prostate gland found from 17th to 20th segments.
- Vasadeferentia along with duct of prostate gland open by male genital pore. Three ducts opens in male gential pore.
- 2 Vasa deferentia and one prostate glands duct. Male genital pore is a group of three pores,

Female reproductive system :
- located In 13th segment is one pair of ovary they are connected to Inter-segmental septa
- The free end of ovary is divided into many lobes. In each lobe are found rows of eggs.
- Eggs - "acropetal order"
- Two oviducts located beneath ovary.
- both oviduct combine and open outside the body by female genital pore in I^st cliteller segment
- Spermatheca - from 6th to 9th segments, one pair in each.
- Two part of spermatheca.

1.Ampula    2.Neck
- A duct is associated with the neck this is known as diverticulum .After copulation sperms are stored in it.
- "Ampulla - Secretes nutritive liquid, this provides nutrition to sperms.

Accessary glands
- One pair in 17th and 19th segment.These opens outside the body at the apex of genital papilla secretion helps to bind to Pheretima during copulation.

Copulation-

Breeding season - At night of rainy season the two Pheretima come closer to each other and tie in opposite direction 18th segment of one comes in contact with 5th '6th segment of other Pheretima.

- A small tube (papilla) is formed around the male genital aperture. These are known as copulation tubes or copulation pegs.
- The copulation tube of one earthworm inserts into spermathecal pore of other.
- The transfer of sperms take place
- Spermatheca gets filled with sperms in one hour.

Cocoon formation :-

Pheretima is protandrous.
A slimy fluid is secreted by glandular cells of clitellum. This then comes in contact with air and forms a pouch around clitellum. This is known as cocoon.
- The eggs comes out from female genital pores and get filled up in cocoon.
- Earth worm wringles back side and cocoon proceeds towards front side.
- When cocoons passes over the spermatheca the sperms gets filled in it.
- Nutritive fluid is given by albumin gland of epidermis.
- Pheretima completely wringles out from the cocoon. fertilization takes place in cocoon only. this is known as "external fertilization".
- Only one egg develops and rest gets destroyed.

Developement:- In 2-3 weeks"
Pheretima gets fully developed and they come out by hatching. They donot have clitellum soon they develop clitellum.

In young earthworm - Setae present in clitellum part.

Cleavage - "unequal" "holoblastic" spiral, determinate.

Gastrulation - Invagination"
"Neurohormone" - Secreted from supra pharyngeal ganglion. these hormones control regeneration of injured tissue, development of clitellum, and other accessory sexual organs, maturation of gametes, cocoon-formation and shedding of gametes etc.

Uses of earthworm :-

Pheretima used to cure rheumatism, piles, asthama, pyorrhoea, impotency, ulcer, jaundice etc.

Losses :- It harms the root of betal

Natural life cycle of pheretima - 3.5 to 10.5 years

Economic Significance of Pheretima & other earthworms -
(1) The burrows makes the soil (land) porous.
(2) They are natural ploughmen of land. So they are friends of farmers.
(3) They act as bait for fishing process.

Harms caused by earthworms :-
(1) Increases soil erosion, Pheretima elongata - harms the roots of betal plant. Some earthworms also harms the plant of paddy and "elaichi plants - e.g. "Malabaria padudicola"

(2) A sporozoan protozoa - Monocystis - resides in seminal vesicles and testes as a parasite.

(3) Acts as hosts for transmission of some parasites (e.g. - gape worm of chickens )

(4) Some times they get burried with dead bodies thus they bring up the infection and harmful microbes with them on the surface.

RESPIRATION

"Cutaneous respiration"

Skin :- Well adapted for cutaneous respiration, it is thin moist, highly vascular. A dense network of capillaries is present.

Normally - Aerobic respiration . But can also live without oxygen for 30 hrs.

Blood Vascular System [B.V.S.]

Closed - Vascular system

True blood vessels present.

Blood - Red colour (haemoglobin present) Hb - dissolved in plasma

R.B.C. - absent (nonchordata) this Hb is knwon as "erythrocruorin" or "inter cellular" Hb.

Dorsal Vessel - "Main blood vessel. from the 3rd to last segment at dorsal side of alimentary canal and it can be seen externally.

Wall of dorsal vessel is muscular and lumen have valves. flow of blood from posterior to anterior side.

Dorsal Vessel :- This function as "Collecting Vessel" from last segment to 14th segment.

- In each segment two pairs of vessels arise from alimentary canal. They are "dorso intestinal vessels". these collect blood from alimentary canal and take blood to dorsal vessel.

- dorsal vessel in anterior part - "distributing vessel"

- 3rd Segment - It gets branched into three branches. They supply blood to the wall of alimentary canal of first
three segments.

- In 3, 4, 5, 6, 8th segment dorsal vessel gives out a pair of vessels. These supply blood to the wall of alimentary canal.

(2) Ventral - vessel - from second to last segment present on the ventral side of alimentary canal.

Wall - non muscular

Valve - absent

Blood flows from - anterior to posterior side

Ventral Vessel - Is a "distributing vessel" in its whole length.

- V.V. in each segment gives out one pair "ventro integumental vessel". These (VIV) supply blood to body wall and related structures.

- septa, nephridia, gonads, etc. get the blood supply.

- After 13th segment V.I.V. supplies blood to the posterior segments.

- After 13th segment a small branch originates from the VIV. This is "septo nephridial" Vessel

This supplies blood to "Septal nephridia" and Inter Seg. septa.

- After 13th segment in each segment one "ventro - intestinal vessel" comes out from V.V. which then supplies blood to alimentary canal.

- V.V. receives blood from dorsal vessel

- the blood comes from dorsal vessel to ventral vessle by 4 pairs of "lateral Hearts".

Hearts :

- 7, 9, 12, 13. lateral hearts (4 pair)

- 12, 13 - "Lateral oesophageal heart"

The hearts are muscular and has valves.

(3) Subneural vessel-

- This is from last segment to 14th segment

- Ventral side of nerve chord

- In 14th segments it branches into two parts

- These are "lateral oesophageal vessel"

Both the branches are situated upto the 2nd segment in the lateral side of alimentary canal.

- S.N.V. and L.O.V. do not have muscular wall.

- They are without valves and blood flows from forward to backwards.

- They act as "collecting vessel" along whole length of animal.

- L.O.V. - Collects blood from walls of alimentary canal and body wall.

- S.N.V. - collects blood only from body wall

- S.N.V. - one pair of commisural vessel comes out from each segment. This supplies blood to "dorsal vessel".

Commisural vessel - Collects blood from "Septa" and Septal nephridia.
From each commisural vessel comes out one "Septo - intestinal vessel". They supply blood to alimentary canal.

Commisural Vessel :- is present at posterior face of septa (intersegmental plates)

(4) "Supra Oesophageal Vessel":-

- In 9-13th segment, on dorsal surface of alimentary canal i.e. on stomach

- collects blood from "gizzard wall". and "Stomach wall".

- In 10th and 11th segment are situated 2 pair of "anterior loops"

- They takes blood from L.O.V. to S.O.V.

- Blood also reaches from L.O.V. to S.O.V. by a dozen loop like ring vessels.

- Blood collected in S.O.V. reach the lateral - oesophageal heart.

From L.O.H. the blood reaches to ventral vessel.

Blood glands :- 3 Pairs of blood glands

- situated on dorsal part of alimentary canal in 4, 5 and 6, segments. They form Hb, blood cell (W.B.C.)

- Leucocytes Hb, blood cell

- Secreted in dorsal vessel.

Lymph glands :-
- On dorsal part of alimentary canal
- from 26th segment to last segment
- 1 (one) pair in each segment
- They form phagocytes and pour them into body cavity.

NERVOUS SYSTEM
Central nervous system :-
(1) Brain ring (2) Nerve cord

(1) Brain ring - This is present around pharynx supra-pharyngeal ganglion in 3rd segment at dorsal surface of pharynx.

- In 4th segment is "sub-pharyngeal ganglion" below the pharynx.

- Nervous strip joins both the two ganglia known as "circum pharyngeal connective"

- A ventral nerve cord comes out from sub-pharyngeal-ganglia. ventral nerve cord extends upto last segment on ventralside of alimentary canal.

- Nerve cord in each segment has one segmental ganglion .

- Ventral nerve cord of Pheretima - double but appears to be single because, it has only one common integument around it.

- 4 giant cells on dorsal surface of nerve cords. these giant cell starts from 4-segment and lasts upto last segment. they are known as "neuro-cords"

This is unique character of Pheretima.

- Pheretima gives rapid - response because of neuro cords.

- All ganglia of Pheretima - formed by fusion of '2 ganglion' in embryonic stage.

Peripheral - nervous system :-
- Supra-pharyngeal ganglia gives out 8-10 pairs of nerves.
- Circum pharyngeal connective - 2 pairs of nerve
- Sub. pharyngeal - 3 pairs of nerves
- Nerve cords - 3 pair nerves in each segment, that arises as:
2 pairs from ganglia and 1 pair directly from nerve cord
- "All nerves" - mixed types of nerves (Sensory + Motor + Adjustor)

Receptor - 3 types.
(1) Epidermal receptors - in epidermis. - tangoreceptors
(2) Buccal receptors - in buccual cavity. - chemoreceptors - ("Olfactory + gustatory)
(3) Photoreceptors- or "ocelli" or"little eyes"

- Maximum in prostomium and peristomium on dorsal side. Each photoreceptor has a large optic cell. Inside optic cell there is a "L" shaped. lens or phaosome.
- A network of nerve fibers is associated to lens. This network is known as "retinella"
- Nerve fibers - combine to form an optic nerve. they take sensory impulse to supra pharyngeal ganglia