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Theory:

Locomotion in earthworm:
Earthworms are invertebrates as they lack bones but contain muscles, and they have a segmented body as their body is made up of rings. They are mostly found in gardens and moist fields. The muscles of earthworms are involved in locomotion.
  • At the time of movement, the earthworm first extends the front part of the body.
  • During this time, the behind portion of the worm remains fixed to the ground.
  • After this, it fixes the front end and pulls the back end forward by shortening the body.
By this, it moves forward by a small distance. By repeating such contractions and relaxations, the earthworm can move through the soil. Its body also secretes a slimy substance to help the movement.
Earthworms fix themselves to the ground with the help of bristles. These bristles are tiny hair-like structures found projecting out under the body known as setae, which provide the grip. These are connected with muscles.
1024px-Earthworm_movement_all.jpg
Movement of earthworm
  
The earthworm finds its food through the soil. It excretes the undigested part of the food consumed from the soil, and it increases the fertility of the soil for the cultivation of plants.
Reference:
https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Earthworm_movement_all.jpg/1024px-Earthworm_movement_all.jpg