Earthworm Anatomy

Earthworms are segmented worms that belong to the phylum Annelida. They have a long, cylindrical body made up of numerous segments called somites. Each segment is externally divided by grooves known as annuli. One of the distinctive features of an earthworm's body is the presence of specialized bristle-like structures called setae.

The Function of Setae

Setae are small, hair-like structures that protrude from the body wall of earthworms. They are primarily composed of chitin, a tough and flexible protein. Setae serve several important functions in an earthworm's life:

1. Locomotion: Earthworms use their setae to move through soil and burrow into the ground. By extending and retracting their setae, they can anchor themselves to the soil and provide traction for forward movement.

2. Sensory Perception: Setae are sensitive to touch and vibrations. They help earthworms detect changes in their environment, such as the presence of predators or the movement of potential mates.

3. Defense: When an earthworm is threatened, it can stiffen its body and anchor its setae into the surrounding soil. This makes it difficult for predators to pull the worm out of its burrow.

Distribution of Setae in Earthworms

In most earthworm species, setae are present in all body segments except for three specific segments. These segments include:

1. The Last Segment: The last segment of an earthworm, also known as the posterior or terminal segment, does not contain any setae. This is the segment closest to the tail or posterior end of the worm.

2. The First Segment: Similarly, the first segment of an earthworm, also known as the anterior or prostomium segment, lacks setae. This is the segment closest to the head or anterior end of the worm.

3. The Clitellar Segment: The clitellum is a swollen, glandular region located near the anterior end of the earthworm's body. It is involved in reproduction and secretes the mucous cocoon in which the eggs are deposited. The clitellar segment does not possess setae.

Answer Explanation

Based on the given options, the correct answer is option D - First, last, and clitellar segments. This means that the first segment, last segment, and clitellar segment of an earthworm do not contain setae. In all other body segments, setae are present and play a crucial role in locomotion, sensory perception, and defense.