Hallucigenia was a genus of wormlike creatures with legs. Similar animals known as velvet worms (Onychophora, ‘claw bearers’) live to this day in decaying tree trunks in tropical forests. But Hallucigenia and many other wormlike animals with legs lived in the sea during the Cambrian and are thought to have been the ancestors of both velvet worms and arthropods.

Hallucigenia had thin legs on the underside and two rows of sharp spines along the back. It fed on microorganisms found on the sea bed.