The key difference between vertebrates and invertebrates is that vertebrates have myelin-forming glial cells (invertebrates do not). The complex, centralized nervous system of the vertebrates, with myelin insulation wrapped around axons like electrical tape is wrapped around wire, is a fundamental difference that separates these two great groups of creatures. (Fields, R. Douglas, PhD. The Other Brain. p 38-40. NY: Simon & Schuster, 2009.)