When Animals Turn Inside Out: The Eversion of Bloodworms

Bloodworms, Glycera dibranchiata, possess an eversible proboscis that normally remains concealed within their bodies but explosively everts if the worm attacks or burrows. How does the bloodworm evert quickly and reliably? In this experimental study, we characterized bloodworm kinematics, pressure, and material properties to estimate the criteria for safe eversion without rupture of the proboscis. We predicted the proboscis can withstand pressures 50 times higher and bending strains up to three times higher than the respective values observed. We also presented a dimensional analysis of eversion, finding that everting animals, from frogs to snails to sharks, do not satisfy Froude's law for equivalence of velocities. Our findings may help inspire the development of pressure-driven soft robots with efficient retraction capabilities.