On the face of it, humans and sea squirts – tubular, rockclinging filter feeders – have little in common. But there’s one fundamental feature that groups us together on the tree of life within the phylum Chordata (‘chordates’). A notochord is a semi-rigid rod that runs the length of the body, providing an attachment site for muscles. In sea squirts, it is found in the free-swimming, tadpole-like larval stage. In vertebrates, it has been largely replaced by the spinal column, although it can still be seen during our embryonic development.
Sea squirts absorb their spine-like notochords.