Acknowledgements 
Grateful acknowledgement is made to the following for permission 
to publish copyrighted figures: University of Chicago Press for 
figures reprinted from Physiological Zoology , the American Physio 
logical Society for figures from Journal of Applied Physiology and 
American Journal of Physiology , and to the editors of the Journal 
of Neurophys iology , Biological Bulletin , Journal of General Physio 
logy , Journal of Cellular and Comparative Physiology , and Science 
