Starfish are marine creatures that appear to be made mostly of arms. But it turns out those body parts aren’t technically limbs.

Scientists at Stanford University in California compared the genes—molecules that carry hereditary information—of one starfish species to the genes of a closely related worm. The scientists looked at the active, or switched on, genes that provide instructions for the development of an animal’s body. To their surprise, the starfish contained active genes for head development throughout its arms and no active genes for the development of a trunk, or central part of the body. Their conclusion: Starfish seem to be all head! The creatures are “best described as just heads crawling along the seafloor,” says Laurent Formery, a biologist who led the study.