NEURO-ENDOCRINE SYSTEMS OF INVERTEBRATES 457 



more clear-cut situation for analyzing the effects of early experience on the later 

 activity of the central nervous system. 



Although the examples were taken from a wide range of animals and of 

 situations, there seems to be a common principle in the use of inhibition. 

 All involve the long-term control of events within the animal. The time scale 

 in these inhibiting events is weeks or months long, rather than milliseconds. 

 And in each case inhibition is used to insure the animal against a course 

 which would mean complete disaster. If the Cecropia silkworm begins adult 

 development before the end of winter, it dies. Premature development of 

 new eggs by a roach would cause a drastic disruption of the reproductive 

 sequence. In the roaches, the corpora allata are innervated by excitatory and 

 inhibitory nerves. If both nerves are severed, the corpora allata do not 

 secrete. This result shows that the absence of excitatory stimuli is usually 

 sufficient to stop hormone release. In normal female roaches, the corpora 

 allata are actively inhibited while the young are being carried. The other 

 examples fit into the same plan. Nevertheless, in these situations it seems 

 undesirable to rely upon the absence of excitatory stimuli to maintain inac- 

 tivity. Perhaps excitable tissues are too unstable and liable to spontaneous 

 discharge for this solution to be acceptable. Instead, inhibition is used as 

 insurance against premature activation. 



This argument is strengthened by considering the control of molting in 

 prawns, where different populations of the same species are dominated by 

 molt-inhibiting or molt-accelerating hormones. This shows that the decision 

 between excitatory or inhibitory control need not be an ancient residue of 

 evolution, but can be selected to fit the circumstances of the animals' life. 



In conclusion, remember that the examples were chosen to show inhibitory 

 hnks in neuroendocrine systems and that a complete review was not attempted. 

 There are undoubtedly many more instances of inhibition remaining to be 

 discovered. Certainly inhibitory links are commonplace in neuroendocrine 

 systems and inhibition plays a vital role in the control mechanisms. 



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