The setae of an earthworm are made up of chitin. Chitin is a tough, flexible, and insoluble polysaccharide that is found in the exoskeletons of arthropods and the cell walls of fungi. It is a type of structural carbohydrate that provides support and protection to various organisms.
Here is a detailed explanation of why the answer is A - chitin:
- Chitin:
- Chitin is a complex carbohydrate that is composed of repeating units of N-acetylglucosamine.
- It is a strong and flexible material that provides structural support and protection.
- It is commonly found in the exoskeletons of arthropods (such as insects, crustaceans, and spiders) and the cell walls of fungi.
- Chitin is also present in the setae of earthworms.

- Earthworm setae:
- Setae are bristle-like structures that protrude from the body of an earthworm.
- They are arranged in rows along the segments of the earthworm's body.
- Setae play a crucial role in locomotion by providing traction and grip as the earthworm moves through the soil.
- The setae are embedded in the epidermis of the earthworm's body wall.
- The setae help the earthworm anchor itself in the soil, preventing it from being pulled out by predators or disturbed by external forces.

- Reasoning:
- Cartilage (option B) is a type of connective tissue found in vertebrates, not in earthworms.
- Pectin (option C) is a type of polysaccharide found in the cell walls of plants, not in earthworms.
- Cuticle (option D) is a waxy, protective layer found on the surface of the epidermis in many organisms, but it is not directly related to the composition of earthworm setae.
- Chitin (option A) is the correct answer as it is the main component of earthworm setae, providing strength and flexibility to these bristle-like structures.
Therefore, the correct answer is A: Chitin.

A