Annelids are bilaterally symmetrical triploblastic animals with metamerically segmented bodies. There exists a true schizocoelic coelom for the first time are annelids. The septa divide the coelom into compartments. Since the coelom is fluid-filled, water pressure is produced, acting as a hydrostatic skeleton.
Locomotion in Annelids (Hydrostatic Movement)
The coelom, the fluid in the coelomic space, and the surrounding musculature make up the hydrostatic skeleton in annelids. The volume of the coelomic fluid is constant. In general, any muscle contraction in an annelid’s body wall would result in a rise in hydrostatic pressure, which would then cause flaccid muscles to stretch.
- When one set of muscles in annelids contracts in a circular and longitudinal pattern, the other set of muscles stretches in response.
- The circular muscles of a segment relax as the longitudinal muscles of the segment contract, and because the coelomic fluid is incompressible, the segment shortens but thickens.
- The protruding setae simultaneously aid in the worm’s anchoring to the substrate.
- When the opposite occurs, the segments become long and thin, the setae are withdrawn and the body moves forward. This is because the circular muscles contract and the longitudinal muscles relax.
Nevertheless, the activities of contraction and relaxation are restricted to a small portion of the body, with the wave of contraction and relaxation moving from one end to the other. This results in the animal’s progression. It has been found that earthworms take about 1 minute to travel 25 cm by this method of locomotion.
Locomotion in Leech
Hirudinea has a more developed body musculature and greatly reduced coelom than polychaete and oligochaete annelids. Animals like leeches (Hirudinea) have suckers that can cling to the surface. The substratum is connected to the posterior sucker. The body lengthens as a result of the wave of circular contraction and is extended forward. Now the anterior sucker is attached and the posterior sucker is released. The body shortens as a result of the longitudinal contraction, and the posterior sucker is brought forward. This is done repeatedly, which causes the leech’s characteristic crawling motion.
Also Check: Diagram of Leech
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