Introduction to Earthworm

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Classification:

Kingdom: Animalia

Phylum: Annelida

Class: Oligochaeta

Order: Opisthopora

Genus: Pheretima

Species: Posthuma

Habit/Habitat of Earthworm:

Earthworms are mostly found in the cosmopolitan, terrestrial, temperate and tropical region.They prefer Loomis soil.It is mostly found in the garden, irrigated land, forest land. They are also sometimes found in sandy soil which is rich in humus. They avoid clay and acidic soil. They can dig up to 2m below the soil. They are positive to geotaxis and negative to chemotaxis.But in the case of food, they are positive to chemotaxis. They are also known as the nocturnal creature as they are active at night.

The segment other than the first segment is called metamers and the first segment is called peristomium. They prepare their burrows by digging the soil with the help of prostomium and swallowing the soil and after that, they plaster it with the help of faeces, mucus, and lime. Lime is prepared at the stomach of the earthworm.

Earthwormmostly breeds during a rainy season, mostly July-October. They are a hermaphrodite creature, but cross -fertilization takes place because they are protandrous.

Morphology of earthworm:

External features(morphology) is the external structures which are visible from outside.The important external features are discussed below:

Shape and size:

The body of an earthworm is cylindrical and elongated. It is a symmetrical animal. It's anterior end is tapering and the posterior end is more or less blunt. It is about 3-5 mm in width and 15 cm long.

Colouration:

It is slimy to touch and appears brown due to the presence of porphyrin pigment in its body wall. Dorsal surface of the earthworm is shining and has a dark mid-dorsal line. Ventral surface of the earthworm is pale in color.

Segmentation:

The body of an earthworm consists of about 100-120 small tiny like segments called metamers. The external segments correspond with the internal segments. So, the segmentation of earthworm is also known as metamerism or metameric segmentation.

Setae:

They are S-shaped and yellowish in color. They are about 80-120 per-segment in number. They are called locomotory organ and assist in locomotion. It is made up of chitin and is arranged in perichaetine which is also called ventrally tical arrangement.

Clitellum:

The clitellum is a thick bond/griddle like structure form by body wall in segment 14,15 and 16.It is secretory in the function which secretes mucus, albumen, cocoon.According to the position of clitellum, the body can be divided into three parts they are:Pre clitellar region(1-13), clitellar region(13-17), post-clitellar region(17-100/120).

Body wall:

The body wall is thin, soft, elastic, delicate and moist in nature formed of different layers. The outermost part, cuticle, is thin, elastic made of collagen, gelatin, and polysaccharides. It is a protective membrane and protects the earthworm from chemical and mechanical injuries. The epidermis is followed by circular and longitudinal muscles. Setae are found between longitudinal muscles which are operated by protractor and retractor muscles.The innermost layer is formed of squamous epithelium called coelomic epithelium.

Functions of body wall:

• It plays an important role in locomotion.
• It protects from mechanical injury.
• The nutritive substance is secreted.
• It helps in respiration.
• It helps in the reception of stimuli.
• It gives shape to the body.

Colon:

It is a body cavity present between the body wall and gut wall. It is filled with coelomic fluid: alkaline with protein, salt, and corpuscles.

Coelomic fluid:

• It plays the role in locomotion as it acts as a hydraulic skeleton.
• It plays a role in respiration.
• It helps in distribution of food from one chamber to another.
• It helps in protection as it absorbs external shock and phagocytizes bacteria of the soil.

Septa:

Partitions present between chambers from the body wall to gut wall is known as septa. In first 4/5 segment, conical anterior 6 septa are present, in 9/10 segment, there are no septa anterior, 9 is without apertures. After 9 septa, the septa has apertures which are provided with sphincter muscle which can close the septa when required or in the case of contraction.

External apertures of earthworm:

Following apertures occur upon the body surface of the earthworm.They are:

Mouth:

It is crescentic aperture lying on the ventral side of a first segment called peristomium and dorsal to it lie prostomium.

Anus:

It is present on the last segment called anal segment which is in the vertical slit-like aperture. It is small in size.

Male genital pore:

Male reproductive bodies are discharged through a pair of crescentic apertures located ventrolaterally upon 18^th segment.

Female genital pore:

A single, minute female genital pore is located in the mid-ventral line of the 14^th segment through which the female reproductive bodies are discharged.

Dorsal pores:

It starts after 12 segments and from this pore coelomic fluid goes out which helps to make surface moist. It lies after 12 segment mid-dorsally in intersegment groove except the last segment.

Nephridiopores:

Except first two segments, they are found in all segments. There is numerous minute nephridiopore in a body wall.These pores are the apertures of integumentary nephridia through which metabolic wastes of the body are removed.

Spermathecal pores:

They are ventrolateral apertures intersegmental present in between segment 5/6, 6/7, 7/8, 8/9 through which spermatozoa enters the spermatheca.They store sperm during copulation.

Genetical papillae:

It is a conical elevation present in segment 17 and 19 a pair each and helps in temporary attachment during reproduction. These are the most prominent structures on the ventral side of a body.