 Norepinephrine has been reported to exist in a wide variety of animal species , including protozoa , <unk> and cnidaria ( jellyfish and related species ) , but not in ctenophores ( comb jellies ) , whose nervous systems differ greatly from those of other animals . It is generally present in deuterostomes ( vertebrates , etc . ) , but in protostomes ( arthropods , molluscs , flatworms , nematodes , annelids , etc . ) it is replaced by octopamine , a closely related chemical with a closely related synthesis pathway . In insects , octopamine has alerting and activating functions that correspond ( at least roughly ) with the functions of norepinephrine in vertebrates . It has been argued that octopamine evolved to replace norepinephrine rather than vice versa ; however , the nervous system of amphioxus ( a primitive chordate ) has been reported to contain octopamine but not norepinephrine , which presents difficulties for that hypothesis . 
