Myelin insulation is formed by oligodendrocytes in the brain and spinal cord, and by Schwannglials in the nervous system outside the brain and spinal cord. (Fields, R. Douglas, PhD. The Other Brain. p 22. NY: Simon & Schuster, 2009.)

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.)