Anatomy of earth worm

An overview about the earthworm
Earthworm is a terrestrial worm living on top layers of the moist soil. It is one of the farmer-friendly organisms appears in a reddish-brown colour. They form the faecal deposits called worm castings which are highly nutritive for crops. In India, 2 of the most common earthworm species are Pheretima and Lumbricus although there are 13 well-known species. Worldwide, there are  5,500 named species of earthworms.

 

Table Of Contents

1. An overview about the earthworm

2. Systematic Position of earthworms

3. Features of earthworms
      3.1 Locomotion, nutrition and reproduction

     3.2 Regeneration

     3.3 Morphology

     3.4 Segmentation

     3.5 Anatomical overview

     3.6 Importance of Earthworms

 

Systematic Position of earthworms

Kingdom:

Animalia 

Phylum:

Annelida 

Class:

Clitellata

Subclass:

Oligochaeta

Order:

Haplotaxida

Suborder:

Lumbricina

Genus:

Pheretima

Species:

Posthuma


 

Features of earthworms
Locomotion, nutrition and reproduction

The earthworm moves slowly by crawling (creeping). Movement is supported by muscular contraction and relaxation with the help of chitinous setae or chaetae. They prey upon decaying organic matter for their food. The food is digested in the gut and undigested food along with the soil forms the worm castings. Earthworm is a bisexual and monoecious( having only one type of sex organ). Worm primarily breeds during the rainy season. It is protandrous (i.e., male sex organs mature earlier than the female) hence there is no chance for self-fertilization.

Regeneration
It has a great capacity to regenerate even if it is cut into two parts. When it is cut, the anterior half develops into the tail.

Morphology

The adult worm measures around 150 mm long and 3-5 mm wide with its cylindrical body throughout. The dorsal surface looks darker than the ventral surface.

 

Segmentation
The body of an earthworm is characterized by segmentation in which the body is divided into more than a hundred short segments. This is also known as metamerism. The mouth is present in the first segment called peristomium. There is a prominent circular muscular band present in the 14th, 15th and 16th segments called the clitellum. It has a single genital pore at 14th segment mid-ventrally. The male genital pores are present in a pair on the ventral surface of the 18th segment.

 

Anatomical overview
There are 4 layers on the body wall of an earthworm. These are cuticle, epidermis, musculature and coelomic epithelium. The outer surface is a thin, non-cellular layer called the cuticle. The epidermis contains a single layer of columnar epithelium along with secretory gland cells. Muscle fibres are of 2 types with an outer thin layer of circular muscle fibres and an inner layer of longitudinal and thick muscle fibres. The outermost coelomic epithelium consisting of flattened squamous cells that helps to protect the internal organs. Earth worm`s alimentary canal is spread through the entire length beginning from the mouth as the first segment and ends with the last segment with an anal opening. The canal is divided into the Buccal cavity with first -third segments, Pharynx at the  4th segment, oesophagus between segment 5 to -7 and the,  Gizzard between 8-9th segments. The stomach is found in between 10th and 14th segments, the intestine is formed segment continued to 15 segments. From 16th the segment continues till the end to form the anus. 
The skin aids in respiration hence it is cutaneous in its respiration. It is thin, transparent and richly supplied with blood vessels. Earthworms have a closed type of vascular system, consisting of blood vessels, capillaries and heart. The blood flows from the posterior end to the anterior end of the channel. There are 4 pairs of hearts at 7th, 9th, 12th and 13th segments respectively. The excretory organs look like coiled tubes arranged in a segmental fashion. They lie near the inter-segmental septa of segment 15. They have a nervous system made up ganglia arranged segment-wise on the ventral paired nerve cord.

 

Importance of Earthworms
Roundworms are farmer-friendly because of their faecal casts. Casts are formed as non-digested wastes which are highly nitrogenous. The method of farming by using this excreta is called vermicomposting. They are also used in catching fish. Countries like China and Japan use these worms as their food.

 





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