NEUROHORMONES 169 



The neurohumors act mainly at short range and for brief duration, while 

 the neurosecretory materials may be carried via the circulation to distant 

 parts of the organism where they may act over relatively long periods 

 of time. The neurosecretory cells are sometimes organized in systems, as 

 for example in the decapod crustaceans. 



There is increasing evidence in both vertebrates and invertebrates that 

 neurohormones are synthesized, transported, stored, and released accord- 

 ing to a common pattern. The invertebrates provide good examples of 

 grouped neuronal endings that are modified for storage of neurohormones. 

 Some of these grouped endings are analogous to the vertebrate neuro- 

 hypophysis. 



Paper chromatography has been successfully applied in the identifica- 

 tion of the neurohumors in certain groups of invertebrates. Examples 

 are cited. None of the neurosecretory substances of invertebrates has yet 

 been chemically defined although work in this direction is now going on. 



We know little concerning the basic mode of action of the neurohor- 

 mones in the invertebrates. Some of the doubly innervated organs, such 

 as the moUuscan heart, furnish favorable material for the study of certain 

 phases of this problem. Here, and in some other places, there is an indi- 

 cation that the neurohumors may act at cell surfaces to alter permeability. 



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