• Scientific Classification of Earthworm:
  • Kingdom: Animalia
  • Phylum: Annelida
  • Class: Oligochaeta
  • Order: Opisthopora
  • Genus: Pheretima
  • Species: posthuma

Habit and habitat of Earthworm:

• Earthworms are segmented invertebrates, that are reddish brown in color.
• Being terrestrial in nature, it mostly inhabits the upper layer of the moist soil.
• It is also fossorial in nature, i.e. it burrows the soil and lives inside burrows made in moist soil.
• They feed on organic matter present in the soil and the undigested substances are expelled in the form of castings.
• The holes of earthworm can be recognized by the presence of castings termed as pellets.
• Earthworms are generally known as farmer’s friend as the fecal deposits of earthworm helps to increase the fertility of soil and burrowing aids in adequate aeration of the soil.
• They are distributed globally and are ranged from sea level to altitude of 3000m.
• However, it is more abundant during the rainy season.
• The earthworm is nocturnal in nature, meaning it stays active at night.

External morphology of Earthworm

• Mouth:
  • It is crescentic in shape and lies on the ventral side of a first segment i.e. peristomium.
  • Dorsal to it, prostomium is present.
• Anus:
  • It is present on the anal segment, i.e. the last segment.
  • The anal segment lies in the vertical slit like aperture.
  • Its size is small.
• Male genital pore:
  • The male genital pores lie ventrolaterally on 18^th segment.
  • They are a pair of crescentic apertures.
  • The male reproductive bodies get discharged through these pores.
• Female genital pore:
  • A single, minute female genital pore is present in the 14^th segment mid-ventrally.
  • The female reproductive bodies are discharged through it.
• Dorsal pores:
  • They are present after 12 segments except the last segment.
  • Coelomic fluid oozes out from this pore that lubricates the surface of body.
• Nephridiopores:
  • They are present in all segments except first two segments.
  • In a body wall, several minute nephridiopores are present.
  • The apertures of integumentary nephridia represent the nephridiopores.
  • The metabolic wastes are discharged out of the body through these pores.
• Spermathecal pores:
  • They are situated ventrolaterally.
  • They are through intersegmental in nature found in segment 5/6, 6/7, 7/8, 8/9.
  • The spermatozoa enter the spermatheca through these pores.
  • During copulation, these pores store sperm.
• Genital papillae:
  • The genital papillae are the most prominent structures present in the ventral side of the body of earthworm.
  • It is a conical elevation found in segment 17 and 19 a pair each.
  • These papillae aids in temporary attachment in course of reproduction.

Morphological feature of Earthworm:

i. Shape and size:

• The body of earthworm is long and cylindrical almost pointed at both the anterior and posterior end.
• It shows bilateral symmetry.
• The anterior end is tapering and the posterior end is more or less blunt.
• The length of earthworm is about 15cm and the width varies from 3-5mm.

ii. Coloration:

• The body appears reddish brown in color.
• The dorsal region is darker in comparison to other regions, and has median dark line.
• The dark coloration is because of the deposition of porphyrin pigment.
• The dorsal blood vessel forms the median dark line.

iii. Segmentation:

• The body is metamerically segmented, seen with 100-120 similar segments called metameres.
• Both the external and internal segmentation is present.
• The circular groove called annuli forms the external segmentation.
• The muscular partition termed as septa forms the internal segmentation.
• The body can be divided into dorsal, ventral, anterior and posterior regions.
• The anterior region lies in close proximity to the clitellum whereas the posterior region lies far from the clitellum.
• In the anterior end, the mouth and the prostomium is present.
• The prostomium is a lobe that aids as a covering for the mouth and helps in burrowing by its wedge like structure.
• Prostomium functions as sensory part.
• The first body segment is termed as the peristomium that possess the mouth.

iv. Setae:

• Setae are S-shaped chitinous structures and are yellow in color.
• Their number ranges from 80-120 per segment.
• Setae are responsible for locomotion and are present in all segments except the first, last and clitellar segments.
• It is arranged in perichaetine order, i.e. in a ring/circle.
• In the setal sac of the body wall, setae are embedded.
• The setae are operated by 2 sets of muscles:
• A pair of protractor muscles (contraction results in extension of sac)
• Single retractor muscle (contraction results in withdrawal of setae)
• Ventral setae are used for crawling on the ground and lateral setae are used while moving in burrows.

v. Clitellum:

• The clitellum is thick girdle like structure formed by body wall.
• It is glandular in nature and pinkish in color.
• It is present in 14^th, 15^th, and 16^th segments.
• It secretes mucus, albumen and during breeding season, it secretes cocoon.
• On the basis of position of clitellum, the body can be divided into three regions that are:
  • Preclitellar region (1-13)
  • Clitellar region (14, 15 and 16)
  • Postclitellar region (17-last)

vi. Body wall:

• The body wall when divided from the surface inwards, comprises of cuticle, epidermis, muscular layers and coelomic epithelium.
• Cuticle:
  • It is thin and elastic in nature.
  • It is non-cellular and is double layered.
  • It is a protective membrane composed of collagen fibers and polysaccharide secreted by the underlying epidermis.
  • It consists of several pores through which the epidermal glands open outside.
• Epidermis:
  • It is cellular in nature.
  • It is a single layer that possess 4 different types of cells lying on basement membrane.
  • Supporting Cells: It forms main bulk of epidermis (provides nutrition to embryo).
  • Gland cells has two types: Mucus cells (mucus secreting cells) and albumen cells (secret albumin abundant in clitellum)
  • Basal Cells: They are found at inner sides of glands and supporting cells (also called replacing cells).
  • Receptor Cells
• Dermis or Muscular layer:
  • It lies just below the epidermis.
  • It has two types of muscles: circular muscles and longitudinal muscles.
  • Circular Muscles:
    • It is an outer thin layer of muscles found in whole body wall.
    • Pigment cells, connective tissue, nerve fibres and blood capillaries are found scattered in them.
    • The length of the earthworm increases by the contraction of these muscles while the thickness decreases.
  • Longitudinal muscles:
    • It is the thick layer of muscle fibres present beneath the circular muscle layer.
    • It occurs in the form of bundles in the body wall.
    • The earthworm shortens in length by the contraction of these muscles but increases in thickness.
• Coelomic epithelium:
  • It is the innermost layer of the body wall.
  • It is known as somatic peritoneum or parietal layer.
  • It is single-celled layer with small nuclei in each cell.

vii. Septum

• In Pheretima posthuma, septa are present in all segments except first four segments and 9/10 segments.
•  The first Septum lies between 4/5 and is thin, membranous and oblique.
• The next four septa are thick, muscular and obliquely placed between 5/6, 6/7, 7/8 and 8/9.
• The first nine septa (4/5,5/6, 6/7, 7/8, 8/9, 10/11, 11/12, 12/13, and 13/14) are complete i.e. they do not have any apertures.
• The remaining septa (14/15, 15/16, 17/18, and so on upto the posterior end) are incomplete i.e. they have perforations.

viii. Coelom

• The body cavity is true and schizocoelous, i.e., formed by the splitting of mesodermal band and strip.
• Dorsal pore of earthworm links externally with the coelom.
• The intersegmental septa divide the coelom of earthworm into compartments.
• The coelomic fluid of earthworm is alkaline and colorless fluid.
•  It contains following 4 types of coelomic corpuscles besides water, salt and proteins.
• Amoebocytes/ phagocytes/granulocytes/ eleocytes:
  • These are small, numerous in number, spheroidal in shape, with a number of petal-like pseudopodia and a large nucleus.
  • These are also termed as Phagocytes (engulfing excretory wastes) as they consist engulfing foreign materials like bacteria in cytoplasm.
  • Because of large spherical structure, concavity & a small nucleus and with many food granules and fat droplets, amoebocytes are also called Granulocytes.
  • These are probably nutritive in function.
• Mucocytes:
  • These are elongated cells with fan like process.
  • These are amoebocytes, that are modified to become elongated and vase-like with nucleus at narrow end and the other end expanded like a fan (fan shaped)
• Circular nucleated cells (leucocytes):
  • These are small, flat, circular, with a large nucleus and clear cytoplasm.
  • These are lesser in number (10%approx).
  • Chloragogen cells/Chlorocyte cells/eleocytes/yellow cells:
  • Chloragogen cells are small, star-shaped with small nucleus.
  • These cells are analogous to liver of vertebrates.
  • These are both nutritive and excretory in function.
  • It is concerned with storage of reserve food and deamination of proteins and formation of urea.
  • Coelomic fluid in Pheretima posthuma plays role as a hydraulic skeleton thus supporting in locomotion.
  • Hemoglobin is absent.
  • The coelomic fluid is secreted by partial peritoneum continuously produced and lost.
  • When earthworm is dipped in spirit, a white milky fluid called coelomic fluid is ejected through the dorsal pores.
  • The lymph glands secrete amoebocytes and blood corpuscles in Pheretima.

Summary Points on Earthworm morphology

• Earthworm belongs to class Oligochaeta.
• Peristomium is the first segment of earthworm.
• Clitellum is located in 14^th, 15^th, and 16^th segments.
• 2 pairs of genital papillae are found in 17^th and 19^th segment.
• Spermathecal pores lie in 5/6, 6/7, 7/8, 8/9 segments.
• Earthworm is hermaphrodite in nature.
• Porphyrin is skin pigment that gives dark coloration.
• The body cavity of earthworm is schizocoel.
• Sperms are stored in spermathecal pores during copulation.

Earthworm: habit, habitat, external feature and morphology

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