As the brain cells and muscles evolved only once during the evolution of animal kingdom, all animals share the similar basic pattern of nervous system.
Neuron is the functional unit of nervous system. The classification of evolutionary relationships among the nervous systems of animal lineages progress from the simplest nerve net to a bilaterally symmetrical and segmented system of neurons. The cell bodies of group of neurons form ganglia. All animals with the spinal cord belongs to the phylum Chordates.
Thus, the blanks can be filled with nerve net; bilaterally symmetrical; ganglia; Chordates.