Earthworm - Locomotion And Body Wall

Locomotion in soft-bodied invertebrates relies on structures suited to their habitat, and NCERT describes the earthworm's mechanism in detail. Each body segment of Pheretima, except the first, last and clitellar segments, bears numerous tiny S-shaped chitinous bristles called setae, which can be extended or withdrawn; by anchoring the setae against soil particles and using alternating contraction of circular and longitudinal muscles, the worm pulls its body forward through the burrow. Parapodia are flat, paddle-like lateral outgrowths used for swimming in marine polychaetes such as Nereis, not present in the earthworm. Tube feet are the hydraulic locomotory organs of echinoderms like the starfish, operating through a water vascular system, and have no place in annelid burrowing. Cilia are microscopic hair-like projections used for movement in protozoans and some larvae, far too small to grip soil for earthworm locomotion. As stated in NCERT Class 11, Chapter 7 (Structural Organisation in Animals), setae help in locomotion of the earthworm. A mechanical check confirms that soil-gripping bristles correspond uniquely to the setae.