Almost all groups of multicellular animals have been investigated to find out if neurosecretory cells are present in them. Such investigations have shown that these cells are located either in brain or ganglia.
Figure: Brain neurosecretory cell -corpus cardiacum -corpus allatum system of a generalized Insect.
However, discrete neurosecretory systems made of neurosecretory cells with their axonal tracts and axon termini forming neurohemal organs have been found among non-chordates only in certain groups. These systems are best known among arthropods, especially in insects and crustaceans.