Question

Assertion: Setae are absent in clitellum.
Reason: Setae helps in locomotion.
A. Both assertion and reason are correct and reason is the correct explanation of assertion.
B. Both assertion and reason are correct but the reason is not the correct explanation for the assertion.
C. Assertion is correct but the reason is incorrect.
D. Both assertion and reason are incorrect.

Answer 1

Hint: Earthworms have a segmented chitinous body. They consist of setae all over their body. They usually inhabit the soil. They are important to the soil as they convert the dead and decaying matter into inorganic form and provide nutrition to the soil. The clitellum is a ring-shaped structure which is present in the segments of earthworm.

Complete Answer:
- Earthworms are also known as farmers' friends. They are terrestrial invertebrates which belong to the phylum annelids. They generally feed on organic matter and are found in the soil. Pheretima and Lumbricus are the earthworms found in India. They have a tube-like segmented body.
- A dark line of blood vessels is present on the dorsal side of their body while genital openings are seen on the ventral side of their body. In a matured earthworm, the segments 14-16 consists of glandular tissue known as clitellum. This clitellum helps to differentiate between mouth and tail.
- So, with respect to clitellum, their body can be divided into three segments- preclitellar, clitellar and postclitellar. It can be present in a single layer such as in microdrile earthworms which causes a lesser number of egg production of eggs. If it is present in multi layers such as in megadrile earthworms are able to produce more eggs.
- In earthworms it can be seen when they mature sexually. When they are ready to mate, the clitellum appears orange. In case of leeches, clitellum is used for sexual reproduction and for secretion of cocoon for eggs.
- It is a part of the reproductive system in subgroups of annelids known as clitellates. For the secretion of cocoon for eggs, it secretes a viscous fluid.
- The body of earthworm is ring shaped which gives a curved shape to the body of the earthworm. They consist of chitinous structure which helps in respiration through the skin. These chitinous structures are called as setae or chaetae. These segments help in locomotion as they are embedded in the skin of earthworms. The setae are bound to the substratum firmly. Thus forming a tough body structure which helps in locomotion and burrowing.

The correct answer is option (B) which states that the assertion and reason both are correct but the reason is not the correct explanation for the assertion.

Note: The locomotion in earthworm is due circular and longitudinal muscles and setae. The setae help in holding the soil while moving. The circular and longitudinal muscles help in contracting the body while keeping setae in place.