Peristaltic waves are radial contractions and relaxation of muscles propagating down a tube or cylindrical structure. In the case of Bonellia viridis, the waves start at the anterior end all the way down the posterior end. It helps them transport water and therefore create a flow into their burrow which is open on both ends (Schembri et al. 1977). As the water gets renewed, they get oxygen as their oxygen uptake is done by diffusion throughout the body.
This movement also allows them to go through narrow holes, turn in their burrow or even move when taken out of their burrow (Schembri et al. 1977).