The Cordata or Cordata episode refers to all species, such as vertebrates or Vertibrata, and certain species that belong to the invertebrates or Invertibrata . The Greek word chorde means rope or string of musical instruments. At some point in the life cycle of this group of animals, there is a stable organ called notochord, the structure of which is similar to that of a rope. So the name of this episode is Cordata. In vertebrate animals, it is this notochord that causes the vertebrae to form in adulthood. The number of species in the Cordata period is about 45,000.