Earthworm: Shape & Body Structure (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.