Locomotion of an Earthworm

A cascade of muscle contractions powers an earthworm's locomotion through the soil.

Earthworms are composed of a series of segments, each with its own inner circular muscles and exterior longitudinal muscles. Earthworm movement begins with a contraction of the inner muscles nearest to the worm's head, setting off a ripple of contraction and expansion that propels the worm through the soil.

Muscles and Skeleton

The earthworm has a hydrostatic skeleton, fluid that resists compression from external pressure. Each segment is filled with fluid and, during movement, the alternating muscle contractions force the hydroskeletal fluid toward its posterior end. When the longitudinal muscles contract, the segment shortens and balloons from the pressure on the hydrostatic fluid. When the circular muscles contract, the longitudal muscles relax and the segment flattens and elongates. An earthworm performs seven to 10 of these contractions every minute.

Setae

Setae are stiff hairs on each earthworm segment that extend and grip the surrounding soil as the worm's longitudinal muscles force fluid into the segment. As the segment contracts, the setae help push the worm forward, then retract until the next wave of expansion and contraction.

Mucous

Each earthworm segment has mucous-secreting glands that lubricate the worm to help it glide through the soil. The mucous also binds soil particles to prevent the burrow from collapsing.

References

• Bioinquiry: Earthworm Locomotion
• University of Florida IFAS Extension: EDIS: Earthworm Biology
• University of North Alabama: Support Systems and Locomotion: Hydrostatic Skeletons
• West Side New York Site: The City Naturalist - Earthworms
• San Francisco State University: Annelids - Coelomates

Resources

• Natural Resources Conservation Service: Soil Biology: The Living Soil: Earthworms