Movement in Earthworms
Earthworms belong to the phylum Annelida and are commonly found on moist soil and vegetation. They have a hydrostatic skeleton characteristic of most cnidarians, flatworms, nematodes and annelids. These animals can control their body form and movement by using muscles to change the shape of the fluid-filled compartments.
Earthworms move by first squeezing the circular muscles of each segment of their bodies to extend themselves forward, gripping the ground surface using bristles, then contracting their longitudinal muscles so that their bodies are pulled up to the front (see Figure 7.7).
Contraction of the circular muscles extends or elongates the body while contraction of the longitudinal muscles shortens and anchors it. This process can occur concurrently at several places along the earthworm’s body. Hence , the earthworm propels itself through the soil and along the ground by repeatedly contracting and extending its muscles.
This alternation of circular and longitudinal muscle contraction waves is known as peristaltic locomotion, a common locomotor pattern in elongated, soft-bodied invertebrates.