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Difficulty: MediumSupport Structures and Locomotion

Arrange the following physiological and mechanical events in the correct sequential order during the peristaltic movement of an earthworm, starting from the initial elongation of anterior segments.

  1. 1Contraction of circular muscles and relaxation of longitudinal muscles in anterior segments
  2. 2Protrusion and anchoring of anterior chaetae into the substrate
  3. 3Contraction of longitudinal muscles and relaxation of circular muscles in anterior segments
  4. 4Retraction of posterior chaetae to allow forward displacement of rear segments

Answer

The correct sequence of peristaltic locomotion in an earthworm is: contraction of circular muscles causing anterior elongation, protrusion and anchoring of anterior chaetae into the substrate, contraction of longitudinal muscles causing body shortening, and retraction of posterior chaetae allowing rear displacement.
In earthworm hydrostatic locomotion, movement relies on alternating waves of antagonistic muscle contraction acting against fluid-filled coelomic compartments. Elongation is initiated by circular muscle contraction. Next, anterior chaetae anchor the extended front to the ground. Then, longitudinal muscles contract to shorten the segments and pull the posterior body forward while posterior chaetae retract to reduce drag.

Step-by-Step Solution

1
Identify the initial muscular contraction responsible for body elongation.
Circular muscles contract while longitudinal muscles relax, extending the anterior body forward.
Hydrostatic pressure pushes coelomic fluid forward when circular muscles exert compressive force on the segment walls.
2
Determine the anchoring step following elongation.
Anterior chaetae protrude and anchor into the substrate.
Anchoring prevents backward slippage when the pulling force is subsequently generated.
3
Identify the muscular contraction responsible for pulling the rear body forward.
Longitudinal muscles contract while circular muscles relax in the anterior region.
Longitudinal muscle contraction shortens and swells the segments, drawing the unanchored rear segments forward.
4
Identify the release step at the posterior end.
Posterior chaetae retract from the soil.
Releasing posterior friction enables the hind portion of the organism to slide forward freely toward the anchored front.

Key Concept

Hydrostatic skeleton and antagonistic muscle action in annelid peristaltic locomotion
Estimated Time:1m 0s
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Topics

Support Structures and Locomotion

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