Earthworm: habit, habitat, external feature and morphology

**Introduction to Earthworm (Pheretima posthuma):**

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.