Diaphragm is a membrane that separates thoracic cavity from abdominal cavity. It is present only in mammals. All other chordates do not have diaphragm as their body cavity is not divided into thoracic and abdominal cavities.

Chordates are coelomate animals having a true coelom, entercoelic and shizocoelic in origin. Pharyngeal gill slits are present at some stage, may or may not be functional. Nerve cord is dorsal and tubular.