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Morphology of an Earthworm

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Morphology of an Earthworm - Lesson Summary

Earthworms are small, reddish-brown worms crawling on the soil or decaying leaves. They live in the upper, moist layers of soil. Earthworms have a long, cylindrical body that is divided into similar segments. In India, there are two common species of earthworms – Pheretima and Lumbricus – which measure 10 to 15 centimeters in length. The body of an earthworm can be dorso-ventrally differentiated. The dorsal surface has a dorsal blood vessel that runs longitudinally along the body, while the ventral surface has genital openings or pores.
The body of an earthworm can be differentiated anteriorly and posteriorly. The anterior end is distinguished by a wedge-shaped structure called the prostomium, it is a sensory organ and also helps to open cracks in the soil and facilitate the movement of the earthworm. The anterior end also bears the mouth, which is attached to the first body segment called the peristomium, it is covered by the prostomium.
The body of a mature earthworm has a thick, dark band called the clitellum. This band is formed around the fourteenth to the sixteenth segments due the secretion of glandular tissue and divides the body into three parts – the preclitellar, clitellar and postclitellar segments.
The fifth to the ninth segments of the body have four pairs of spermathecal apertures in the ventro-lateral sides of the intersegmental grooves. The fourteenth segment has a  female genital pore, while the eighteenth segment bears a pair of male genital pores. Each segment of an earthworm has minute pores called nephridiopores on the ventral side, which serve as excretory organs.
The earthworm’s body is rough to the touch due to the presence of tiny S-shaped bristles called setae which are embedded in the epidermal pits. Each segment of the body, except the first, last and the clitellum, has four pairs of setae which help in locomotion.
Earthworms are known as ‘friends of farmers’, because their burrowing activities make the soil porous and aerated, which facilitates the development of roots in plants.
Besides, they also play an important role in decomposing organic matter to humus, improving soil fertility. In recent years, earthworms have been extensively used for vermicomposting. 


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