Which muscle layer contracts when a worm actively lengthens and becomes thinner during movement?
Answer
Circular muscles
Locomotion in worms, which rely on their hydrostatic skeleton, is controlled by two primary, opposing muscle layers lining the body wall: circular muscles and longitudinal muscles. When the circular muscles contract, they squeeze the body circumferentially, forcing the internal fluid to redistribute and causing the worm's body to become longer and significantly thinner. This lengthening action is crucial for extending the front end of the worm forward or for efficiently pushing through tight spaces in soil or sediment during burrowing activities.
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