Explanation:
Earthworms have a hydrostatic skeleton, which means they use fluid pressure to control their movements. The circular muscles and longitudinal muscles work together to create movement in the earthworm's body.

Setae:
Setae are bristles that are found on the body of an earthworm. They are used to provide traction and grip on the surface the earthworm is crawling on.

Circular Muscles:
Circular muscles are found in the body wall of the earthworm. When these muscles contract, the diameter of the earthworm's body decreases, causing it to become longer and thinner.

Longitudinal Muscles:
Longitudinal muscles run the length of the earthworm's body and are found just under the skin. When these muscles contract, the body of the earthworm becomes shorter and fatter.

Locomotion:
The earthworm's method of locomotion involves alternating contractions of the circular and longitudinal muscles. The circular muscles contract first, causing the earthworm to become longer and thinner. The setae grip the surface and prevent the earthworm from slipping. Then the longitudinal muscles contract, causing the earthworm to become shorter and fatter. This movement pushes the earthworm forward. The earthworm repeats this process of contracting and relaxing its muscles to move forward.

In conclusion, earthworm locomotion occurs with the help of setae, circular muscles, and longitudinal muscles. The setae provide traction and grip, while the circular and longitudinal muscles work together to create movement in the earthworm's body.