All chordates share four basic features.
Chordates derive their name from one of their synapomorphies , or derived features indicating their common ancestry. This is the notochord , a semi-flexible rod running along the length of the animal. In those chordates which lack bone, muscles work against the notochord to move the animal. The notochord runs beneath the dorsal nerve cord , which is another chordate feature. This is in contrast to organisms such as annelids and arthropods , in which the main nerve cord is ventral.