Anatomy of the earthworm

Earthworms or angleworms are the terrestrial organisms, appears thin and reddish-brown, inhabiting mostly on top layers of the moist soil. They belong to the subclass Oligochaeta, phylum Annelida and are the members of the genus Lumbricus. They are one of the farmer-friendly organisms as they form the faecal deposits called worm castings; a great choice of farmers to grow crops. Worldwide, there are  5,500 named species of earthworms while there are 13 well-known species in India,  among which , the most common earthworm species  are  Pheretima and Lumbricus. The body of atypical worm looks like an open tube, externally segmented with a corresponding internal segmentation. They possess setae-bristle like extensions on all segments. Earthworms  are invertebrates as they lack a true skeleton. However, they retain their structure with fluid-filled coelom chambers that form a hydrostatic skeleton.

Systematic Position of earthworms

Kingdom:

Animalia 

Phylum:

Annelida 

Class:

Clitellata

Subclass:

Oligochaeta

Order:

Haplotaxida

Suborder:

Lumbricina

Genus:

Pheretima

Species:

Posthuma

Features of earthworms

Segmentation

The body of an earthworm is well distinguished due to its segmentation. Metamerism is a process that helps to form more than a hundred short segments. An earthworm has a typical cylindrical tube-in-a-tube appearance with a series of segments called metamerisms. Metamerisms helps to compartmentalize the body. as each organ of the worm is distributed across different numbers of segments throughout its length. An individual segment carries many bristle-like hairs called lateral setae but these are absent in the mouth and anal segments. They aids to anchor the whole body during locomotion. The mouth or the peristomium is located in the first segment. The female worm has a single genital pore at its 14th segment placed mid-ventrally while a pair of male genital pores are found on the ventral surface of the 18th segment. The ventral setae found in the anal region are used to anchor mating through the penetration of the sexual organ into the sexual orifices of their mates.The prominent circular bands of muscles found on the 14th, 15th and 16th segments are called the clitellum.

Anatomical overview

The body of earthworm is made up of 4 layers; cuticle, epidermis, musculature and coelomic epithelium. The outermost surface is a thin, non-cellular layer called the cuticle which helps to protect the worm from becoming dehydrated. The epidermis forms the first nonspecific barrier of earthworms because  the outermost coelomic epithelium consisting of flattened squamous cells help in protecting the internal organs. It is formed by a single layer epithelium along with the basal cells, supporting cells, and secretory cells. The basal cells help in wound healing and graft rejection in addition to phagocytic activity while all others support these basal cells. Throughout its body, the worm has 2 types of muscle fibres; an outer thin layer of circular muscle fibres and an inner layer of longitudinal and thick muscle fibres.

 

Morphology

Depending upon the species, an adult worm measures between 10 mm to 3 meters long and 3-25 mm wide with their cylindrical body throughout. The dorsal surface looks darker than the ventral surface. The dorsal pores and nephridiopores found on the cuticle secrets some fluids to moisturize and protect the whole body.

Locomotion

An earthworm moves with the help of circular and longitudinal muscles in coordination with the setae. The worm can extend its setae out of its body to grab the surface around it. The setae help to anchor the front portion of its body while the contracting the longitudinal muscles which shorten its body. Spontaneously, when these muscles relax, the length of the worm increases. the series of elongation and shortening of the body by these muscles produces locomotion. The movement in an earthworm is more of reflexive rather than neurological, it can be demonstrated by touching it. If you touch an earthworm, it stimulates the subepidermal nerve plexus that are attached into the intermuscular plexus. This can cause the contraction of the longitudinal muscles resulting in writhing movements.

Feeding

A typical earthworm can eat the food that weighs roughly half its body weight every day. They prey upon decaying organic matter found in the moist topsoil,  fruits, berries and vegetables and they are capable of consuming algae, fungi and bacteria found underground. Vermicomposting and distribution of organic wastes on a regular basis help to meet worm`s food and nutrient requirements in a ddition to the provision of  the optimum temperature and moisture necessary for the worm to live. Some species reach the surface and graze on the organic matter present there. The worm casts released here  mix with the soils to provide a good balance.

Reproduction

The earthworm can regenerate by itself even if it is cut into many large segments.  Therefore, they are not necessarily the organisms exhibiting pure sexual reproduction. A variety of species from the past 6 million years exhibited different reproductive behaviours. A typical earthworm is a monoecious organism;  it either has a male or female sex organ. However, some earthworms are hermaphrodites as they possess both male and female sexual organs. They essentially breed throughout the rainy season and are protandrous in nature (i.e., male sex organs mature earlier than the female). Earthworm mate on the surface, often at night. The sexual organs are found in segments 9 to 15. Each earth worm has 1-2  pairs of testes within the reproductive sacs where spermathecae-the internal sacs helps to  receive and store sperms produced by the segments 9-15. Segment 13 help to release ovaries . The ventral setae found inn the anal regions assist in coordinating the mating. 

Body systems

Digestive system

The segmentation of the alimentary canal begins from the mouth and ends at the last segment or the anal opening. The alimentary canal is divided into the buccal cavity located at segments 1-3, the pharynx at the  4th segment while oesophagus lies in the segments 5 -7. The area between the segments 8-9 is called Gizzard and the stomach fits int0 10th - 14th segments while the intestine is formed at segment 15. From its segment 16, the anus continues till the dead end.

Vascular system

Like many other animals, earthworms have a closed vascular system made up of heartblood vessels, and tiny capillaries. The blood flows from posterior end to the anterior end of the body. The worm has 4 pairs of hearts found at 7th, 9th, 12th and 13th segments respectively. The 3 main vessels help in supplying the blood to all organs are the aortic arches, dorsal blood vessels, and ventral blood vessels. The worm has 5 pairs of aortic arches responsible for pumping blood into the dorsal and ventral blood vessels where the dorsal blood vessels carry blood into the frontal region of the body while the ventral blood vessels push blood to the back of the earthworm’s body.

Skin

The outer surface of the worm is thin, transparent,  richly supplied with enormous blood vessels and the main function of the skin is to help in cutaneous respiration.  

Excretory organs

The excretory organs that appear-like coiled tubes are arranged in a segmental fashion. They rest near the inter-segmental septa of segment 15.

The nervous system

The worms have a nervous system made up ganglia arranged in the form of segments around the ventrally found nerve cord. Specialised sensory organs and chemoreceptors help them to respond to stimuli perfectly. The sensory system of the earthworms is present in the anterior portion of the body.

Importance of Earthworms

Agricultural importance

Earthworms burrow through the soil building space for air entry into the roots . The spaces form tunnels allow rain and irrigation water to penetrate into the soil. The worm can burrow up to 6 feet in the soil help in mixing the different layers of soil while breaking the dense granules of soil into fine particles. Worm castings contain nitrogen, phosphorus, and potassium which can be a great addition to the soil.

Role in the food web 

They play an essential role in food chains and ecosystems as they can decompose, break down and recycle the dead matter from plants and animal. It turns many wastes into organically useful products this helps in returning wastes into useful materials it back into the soil. This is how a compost pile works!

Read More

  1. Animal Kingdom

  2. Microbes in human welfare

  3. Saprotrophs 

  4. Autotrophs and heterotrophs   

Questions

  1. Why are earthworms important in farming?

  2. Explain the brief morphology of an earthworm.

  3. Write the role of earthworms in keeping the environment clean.

  4. Explain about vermicomposting.

  5. Explain the reproductive mechanisms of earthworms.

  6. Write the systematic Position of earthworms.

  7. What is metamerism?

  8. Explain the role of segmentation in earthworms.

  9. Describe the vascular system of an earthworm.

  10. Explain the mechanism behind the locomotion of an earthworm.

  11.  What are hermaphrodites? Are earthworms purely hermaphrodites? Explain.

  12. What are nephridiopores?

 

 



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