Morin, J.G. and Hastings, J.W.
Biochemistry of Bioluminescence of Colonial Hydroids and Other Coelenterates.  Journal of Cellular Physiology 77: 313-318 (1971).

Many (but not all) of the bioluminescent systems in coelenterates involve energy transfer from an excited product molecule of the calcium activated photoprotein to a second species, the green fluorescent protein, with emission at 508 nm from its excited state. Although all the luminescent coelenterates studied possess photoproteins, not all of them have the green fluorescent protein. This green fluorescent molecule is localized in the luminescent cells; they can thus be easily distinguished by fluorescence microscopy. The active components occur in subcellular particles; these have been isolated in an active form by homogenization in isotonic (to sea water) salt solutions.