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Earthworm: A complete study material in summerized form; Anatomy, Morphology and organs location



Earthworm: A complete study material in summerized form.


Earthworm, Taxonomic classification, Morphology of earth worms, Anatomy of Earthworm, Digestive system, Reproductive system, Excretory system, Circulatory system, forest of nephridia, Clitellum, earthworm mouth is present in, table of earthworm organs and related segments.

Taxonomic classification of Earthworm (Pheretima posthuma):

Kingdom: Animalia
    Phylum: Annelida
        Class: Oligochaeta
            Order: Haplotaxida
                Suborder: Lumbricina
                    Family: Megascolecidae
                        Genus: Pheretima
                            Species: Posthuma


Habit and Habitat

•burrowing (fussorial) animal living inside burrows made in moist soil.
•nocturnal creature
•comes out of the hole during night for feeding.
•Expell undigested food in the form of small castings(worm casting). Their holes can be identified by the presence of casting called pellets.



Morphology of Earthworm

• Bilaterally symmetrical and metamerically segmented body with 100 to 120 similar segments called metamers.
• Segmentation is both external and internal.
• External segmentation by circular groove called annuli.
• Internal segmentation by muscular partition called septa.
•body color: dark brown or clay color due to porphyrin pigment.
•Dorsal surface of animal is comparatively darker than ventral.

Clitellum:
    -glandular, pinkish, girdle like thickening of body wall.
    -present in 14th,15th and 16th segments.
    -during breeding season, it secretes cocoons.
    -Due to cocoon, body is divided into three segments.
        Preclitellar region(1st to 13th segments)
        Clitellar region(14th, 15th and 16th segments)
        postclitellar region(17th to last segments).

Setae:
- S-shaped locomotary structure made up of chitin.
- Present in all segments except 1st, last and clitellar region.
- Setae are arranged in a circular or ring pattern.
- There are 80-120 setae per ring.

Body wall:
Body wall of earthworm consist of four layers: cuticle, epidermis, muscular layer and coelomic epithelium.

(a)cuticle:
-thin and elastic non cellular double layered protective membrane.
-made up of polysaccharide and collagen fibres.
-presence of numerous pores for gland secretions.

(b) Epidermis:
-cellular, single layer resting on basement membrane.
-consist of four different types of cells.
-- Supporting cells: provides nutrition to embryo.
-- Gland cells: secrete mucus and albumen.
-- Basal cells: also called as replacing cells.
-- Receptor cells: receive stimuli.

(c) Dermis or Muscular layer:
-Situated below epidermis.
-Have two types of muscles.
-- Circular muscles: by its contraction, length of earthworm increases and thickness decreases.
-- Longitudinal muscles: by its contraction, length of earthworm decreases and thickness increases.

(d) Coelomic epithelium:
-innermost layer of the body wall.
-single-celled layer with small nuclei in each cell.

Septum (pl. Septa):
-present in all segments except first four segments and 9/10 segment.
-first nine septa are complete (without any apertures) and and remaining all are incomplete (with perforations).

Coelom:
-True body cavity.
-Schizocoelous.
-Coelom is divided into many compartments by intersegmental septa.
-Coelomic fluid is alkaline and colourless.
-Coelom consist of water, salt, proteins, phagocytes, mucocytes, lucocytes and chloragogen cells(analogous to liver of vertebrates).



Body structures of Earthworm and its distribution.

Body Parts and Structures of Earthworm

Located Segments

Digestive system
Prostomium Attached with 1st segment
Mouth (peristomium) 1st segment
Buccal cavity 2nd -3rd segment
Pharynx 3rd-4th segment
Oesophagus 5th-7th segment
Gizzard 8th-9th segment
Stomach 9th-14th segment
Intestine 15th-last segment
Anus Last segment (pygidium)
Pre-typhlosolar region 15th to 25th segments
Typhlosole 26th to last except last 23rd or 25th segments
post-typhlosolar region (rectum) last 23rd or 25th segment to last
Reproductive System
Clitellum 14th, 15th and 16th segments
Male organs
Testis(2pairs) 10th and 11th segments
Testis sacs (1pairs) 10th and 11th segments
Spermiducal funnels (2pairs) 10th and 11th segments
Semimal vesicles (2pairs) 11th and 12th segments
Vasa deferentia (2pairs) 10/11th to 18th segments
Prostate gland (1pairs) 16/17th to 20/21st segments
Accessory gland (2pairs) 17th and 19th segments
Common prostatic gland (1pairs) 18th segment
Male genital aperture (1pairs) 18th segment
Genital papillae (2pairs) 17th and 19th segments
Female organs
Ovaries (1pair) 13th segment
Oviducal funnels (1pair) 13th segment
Oviducts (1pair) 13th and 14th segments
Female genital aperture (single) 14th segment
Spermathecal pores (4pairs) 5/6, 6/7, 7/8, 8/9 segments
Spermathecae (4pairs) 6th, 7th, 8th, 9th segments
Circulatory System
Lateral hearts (2pairs) 7th and and 9th segments (1pair in each segment)
Lateral oesophageal hearts (2pairs) 12th and 13th segments (1pair in each segment)
Dorsal blood vessel 1st to 13th segments (distributive)
14th to last segments (Collecting)
Lateral oesophageal vessels(1pair) 1st to 13th segments
Subneural vessel 14th to last segments
Supraoesophageal vessel 9th to 13th segments
Anterior loop (2pairs) 10th and 11th segments (1pair in each segment)
Blood glands 4th, 5th and 6th segments
Lymph glands 26th to last segments (1pair in each segment)
Excretory System
Septal nephridia 15th to 2nd last segments
Integumentary nephridia 7th to 2nd last segments
Pharyngeal nephridia 4th, 5th and 6th segments
Nervous System
Nerve ring 3rd and 4th segments
Nerve cord 5th to last segments


Digestive system

•Alimentary canal is straight with 'tube within a tube body' plan.
•It has different parts:

a) Mouth
-Semi circular and present in 1st segment.
-also called as peristomium.
-present below the prostomium.

b) Pharynx
-thick and highly muscular.
-its cavity is divided into ciliated dorsal and non-ciliated ventral chambers.
-Pharynx is the effective organ of food digestion.

c) Oesophagus
- quite thin tube.
-does not posses any glands or muscular folds.

d) Gizzard
-formed by outer layer of circular muscle, single layer of epithelial layer and innermost thick layer of cuticle.
-it acts a grinder for grinding of food.

e) Stomach
-long tube with short narrow cavity.
-glandular chamber.
-it's wall contains calciferous gland whose secretion neutralizes the acidity of soil.

f) Intestine
-has got long fold of epithelium called typhlosole.
-based on typhlosole; it is divided into 3 regions.
i) pre-typhlosolar region= highly glandular and contains intestinal caeca.
ii) typhlosolar region= glandular and highly absorptive.
iii) post-typhlosolar region= absorptive; also called rectum.

g) Anus
-present at last or anal or pygidial segment.
-vertical, silt like, terminal, small aperture.
-for defaecation of casting.

Circulatory System

•Closed type of blood vascular system.
•Respiratory pigment is haemoglobin which is dissolved in blood plasma.
•The main longitudinal vessel has different parts. They are:

a) Dorsal blood vessel
-Largest blood vessel.
-Present above the intestine.
-Contains valve from 14th segment upto last.
-Considered as true heart in earthworm.
-Blood flows: posterior to anterior direction.
-Distributive upto first 13 segments.
-Collectng from 14th segment upto last.

b) Ventral blood vessel
-Valve less.
-Present below alimentary canal.
-It is main distributive blood vessel of earthworm.

c) Lateral oesophageal vessels
-It is paired and located on the both sides of alimentary canal.
-Collects blood from seminal veaicles.

d) Subneural vessel
-Lateral oesophageal vessels unite to form subneural vessel. -located below ventral nerve cord.

e) Supraoesophageal vessel
-Smallest vessel of body.


•Other related organs:
  1. Lateral hearts
  2. Lateral oesophageal hearts
  3. Anterior loop
  4. Blood glands
  5. -syntheaizes blood corpuscles and haemoglobin.
    -associated with pharyngeal nephridia.
  6. Lymph glands
  7. -supposed to produce certain phagocytic cells.


Respiratory System

•No special respiratory organs.
•Respiration through skin ie. cutaneous.
•If skin becomes dry, it cannot respire and dies due to asphyxia.


Excretory System

•Excretory organ: nephridia
•Nephridia are ectodermal in origin.
•Analogous to kidney of vertebrates.
•Present in all segments except first 3 and last segments.
•Based on strucure and location; they are 3 types.

  1. Septal nephridia
  2. -Enteronephric nephridia
    -largest nephridia
    -consist of 4 parts: Nephrostome, Neck, Body of nephridium and Terminal duct.
    -Only nephridia with nephrostome.
  3. Integumentary nephridia
  4. -Exonephric nephridia
    -Smallest, V-shaped and most numerous.
    -heavy number of integumentary nephridia are present in clitellar region, so this region is called as forest of nephridia.
    -lack of nephridiophores.
  5. Pharyngeal nephridia
  6. -Enteronephric nephridia
    -3pairs, present as paired tufts on either side of pharynx.
    -Each tufts contains of hundreds of nephridia.
    -Every nephridia joins to form common pharyngeal nephridial duct.
    -Ducts from 4th and 5th segments open into the pharynx in 4th segment.
    -Duct from 6th segment open into buccal cavity in 2nd segment.


Nervous System

•well developed and metamerically segmented nervous system.
•Presence of three types of nervous system.
•they are:

  1. Central Nervous System
  2. -It includes a nerve ring and a nerve cord.
  3. Peripheral Nervous System
  4. -All PNS nerves in earthworm are mixed type. ie. both sensory and motor.
  5. Autonomic Nervous System
  6. -Presence of only sympathetic nervous system.

Sense organs:
There are three types of sense organs in earthworm. They are:
i) Epidermal receptors (Tangoreceptors):-
-perceive heat and chemical stimuli.
ii) Buccal receptors:-
-perceive smell and taste of food.
iii) Photoreceptors:-
-acts as eyes; judge intensity and duration of light.

Reproductive System

-Monoecious or bisexual or hermaphrodite.
-Protandrus.
-Each testis consists of 4-8 finger like projection.
-Spermiduct funnel is internally ciliated, helps in conduction of spermatozoa.
-Seminal vesicles are also called as septal pouches.
-Seminal vesicles provide nourishment to the sperm.
-Ovary is whitish in color.
-Ovaries are larger than the testis.
-Spermathecae are flask shaped and 4 pairs in number.
-Spermathecum is composed of large compartment called ampulla and small compartment called diverticulum.
-They are for sperm collection from another earthworm during copulation.
-Secretion of accessory glands helps to keep earthworm together during copulation.
-Copulation occurs generally in night of rainy season.
-Two earthworms stick together facing opposite direction.
-Both earthworm receive sperm from one another.
-Fertilization occurs in the cocoon.
-Fertilization is external.
-Cocoon is formed by the secretion of clitellum.
-Cleavage is holoblastic and unequal.
-Development is direct, without larval stage.




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