Question

Rows of S-shaped setae in the body of earthworm are present in all the segments, except

• A. The first segment
• B. The last segment
• C. The first and last segment
• D. The first, last and clitellum

Answer 1

The Correct Option is D

To understand the distribution of setae in earthworms, let's break down the details of their anatomy and function:

1. Setae: Setae are small, bristle-like structures made of chitin. They project from the body wall of earthworms and are used for locomotion and anchoring the worm in the soil. Their S-shape provides strength and flexibility.
2. Segmental Arrangement: Earthworms have segmented bodies, meaning their bodies are divided into a series of repeating units called segments or metameres. Most segments contain setae arranged in specific patterns (typically in rows).
3. Exceptions to the Rule: However, there are a few segments where setae are absent:
   • The First Segment (Peristomium): The peristomium is the first body segment, which surrounds the mouth. It does not have setae because its primary function is feeding, not locomotion.
   • The Last Segment (Pygidium): The pygidium is the terminal segment of the earthworm, containing the anus. Like the peristomium, it lacks setae. Its primary function is excretion.
   • The Clitellum: The clitellum is a thickened, saddle-like region present in sexually mature earthworms. It's involved in reproduction, secreting a cocoon that holds the eggs and sperm. Setae are absent in the clitellum to facilitate the formation and movement of the cocoon.
4. Why These Segments Lack Setae: The absence of setae in these regions is related to their specialized functions. The peristomium needs a smooth surface for feeding, the pygidium for excretion, and the clitellum for cocoon formation. Setae in these regions would interfere with these functions.

Therefore, the rows of S-shaped setae are present in all the segments of an earthworm, EXCEPT for the first segment (peristomium), the last segment (pygidium), and the clitellum.

Answer 2

The correct answer is:

Option 4: The first, last and clitellum

Explanation:

Setae are small bristle-like structures that are used by earthworms for locomotion. Rows of S-shaped setae are found on the body segments of an earthworm.

However, setae are absent in the first segment, the last segment, and the clitellum (the thickened region used for reproduction, typically found in segments 32 to 37).

The absence of setae in these segments helps with the earthworm's movement and reproduction. The clitellum produces mucus for the formation of the cocoon during reproduction, while the first and last segments do not require setae for movement.

Thus, setae are absent in the first segment, the last segment, and the clitellum.