What is the name for animals with backbones?

Vertebrates - fish, amphibians, reptiles, birds and mammals. It feels like the main category of animal because every animal a person is likely to name belongs to it, and that is a spectacular distortion of scale. Vertebrates are roughly five per cent of known animal species. The other ninety-five per cent have no backbone, and most of them are insects.

The defining structure is not really the backbone but the notochord, a stiff flexible rod that appears in the embryo. In us it is largely replaced by the spine and survives only as the jelly in the middle of each intervertebral disc - so a slipped disc is a failure of the oldest piece of vertebrate anatomy there is.

The closest living relatives make the point about how much is not vertebrate. Sea squirts spend their larval stage as a swimming tadpole with a notochord and a nerve cord, then settle head-first onto a rock, absorb their own brain, and spend adulthood as a filter-feeding sac. That animal is a closer relative of yours than a lobster or an octopus is.