Using examples of invertebrate nervous systems how can the process of evolutionary cephalization be described?

Considering the instance of invertebrates it is observed that evolution makes the enhancing of the complexity of the organisms to be accompanied by convergence of nervous cells to special structures for controlling and commanding: the ganglia and the brain. In easy invertebrates, like cnidarians, the nervous cells are not concentrated but they are found dispersed in the body. In platyhelminthes a starting of cephalization with the anterior ganglion concentrating neurons is already verified. In annelids and arthropods the existence of a cerebral ganglion is evident. In cephalopod molluscs the cephalization is even big and the brain commands the nervous system.