Bioluminescence* *Grateful acknowledgement is made for aid to this paper by NSF grant GB-4086 to, and ONR contract Nonr 4246 (00) with, Princeton University, and by the staff of the Kristinebergs Zoologiska Station, Fiskebäckskil, Sweden, who kindly provided helpful library and other facilities during the completion of the manuscript. Appreciation is expressed also for the permissions received from authors and publishers to include previously published figures from the sources indicated.