The nervous system represents tremendous complexity and diversity. However, little light has been shed on the origin of the nervous system and its original function in the evolutionary history. An evolutionary approach is essential for proper understanding of the nervous system. The nervous system in planula larvae of cnidarians, which is the most primitive animal, is much more simple than the diffuse nervous system of their adults, and they might still reveal the past original function the nervous system when acquired in evolution. In this study, I investigated nervous system functions in planula larvae of reef-building corals focusing on peptide neurotransmitters. One neuropeptide family, GLWamide, induced metamorphosis of coral larvae, and another neuropeptide family, GRFamide, inhibited metamorphosis. Coral larvae seem to sense environmental cues to choose places for their future sedentary life in nature. In this process, positive or negative cues may be converted to corresponding internal signals, and the neuropeptides are the candidates for the internal signal molecules acting hormonally to regulate metamorphosis. This is a simple response to environments, and will be the original function of the nervous system. The neoroendocrine will be the ancestral system and controls the basses of life activities even in the higher animals.