Why did the vertebrate organ systems change their location?

The lancelet is the first primitive vertebrate animal. It was from him that the rest of the vertebrates were later formed. Lancelet is primitive in structure. The animals that descended from him later began to hunt and catch prey, it is with this that the strengthening of their muscles and longitudinal skeleton is connected. In this regard, both the organs of locomotion and the sense organs have become more developed in vertebrates. Due to the general complication of organ systems, the structure of the brain became more perfect, then the cranium appeared, which protects the brain. More active movement gave rise to increased blood circulation and a four-chambered heart.