Explanation

Chordates possess dorsal, hollow, fluid-filled nerve cord. It is formed by infolding of a mid-dorsal strip of ectoderm and it generally sinks below surface. It lies above the notochord and outside the coelom, it has a hollow canal running from one end to the other. This dorsal tubular nerve cord persists throughout life in most chordates but few degenerates it before maturity. It serves for the integration and coordination of body activities.