Habit and Habitat: Earthworm is a reddish brown terrestrial invertebrate. It lives in the upper layer of moist soil. Pheretima and Lumbricus are the common Indian earthworms. Earthworms can be easily seen during rainy season.
Earthworm has a long cylindrical body which is divided into more than a hundred short segments or metameres.
Dorsal Surface: There is a dark median mid-dorsal line (along the longitudinal axis) on the dorsal surface of the body. This line marks the dorsal blood vessel, that lies beneath the body wall.
Ventral Surface: Genital openings can be seen on the ventral surface of the body of an earthworm.
The anterior end consists of the mouth and the prostomium. Prostomium is a lobe which serves as a covering of the mouth. It also acts as a wedge to force open cracks in the soil. The prostomium has sensory function as well.
The first body segment is called peristomium or buccal segment. The peristomium contains the mouth.
Clitellum: In a mature worm, segments 14 to 16 are covered by a prominent dark band of glandular tissue. This band is called clitellum. Based on the relative position of clitellum, the body is divided into three main regions, viz. preclitellar, clitellar and postclitellar segments.
Genital Openings: On the ventro-lateral sides of the inter-segmental groves (between 5th – 9th segments) four pairs of spermathecal apertures are situated. These apertures are the passage for entry of spermatophores.On the mid-ventral line of 14th segment, a single female genital pore is present. On the ventro-lateral sides of the 18th segment, a pair of male genital pores is present. Pairs of gential papillae are present on 17th and 19th segments.
There are 5 rows of S-shaped setae in each body segment. The setae are embedded in the epidermal pits in the middle of each segment. Setae are absent in the first segment, clitellum and last segment. Setae can be extended or retracted and facilitate locomotion. The earthworm gets hold of substratum with setae on the anterior portion of the body. After that, it pulls the rear portion of the body ahead. Then it moves forward the anterior portion of the body to move ahead. That is how locomotion happens in earthworm.
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