762 LIGHT AND LIFE 



ferred to were described a half-century ago, and from preserved ma- 

 terial, or if he knows that the skins of certain squids and Crustacea 

 are studded with refractile iridiophores, he will be even more sus- 

 picious. In point of fact, the situation is by no means as unequivocal 

 as one could wish; but to make a long story short, I did eventually 

 assemble enough eye-witness accounts of light production by precisely 

 localized and identified structures to justify, in my opinion, proceed- 

 ing on the assumption that there do exist eye-like organs that produce 

 light. (There is even less evidence concerning what parts of the 

 organs produce the light.) Actually, the uncertainties are greater 

 for presumptive eyes, which have usually been identified only on the 

 basis of looking like eyes and being on the head; but presumably one 

 is justified in relying on homology in regard to such a universally 

 present function as vision. Parenthetically it might be mentioned 

 that the burden of speculation would not have been appreciably eased 

 had the "photophores" actually turned out to be eyes after all, since 

 then the rationale of their presence on tentacles, swimmerets, gill- 

 chamber wall, and perianal skin would come into question, or by 

 demonstrating that the organs in question were neither eyes nor 

 photophores, in which case their ostensibly dioptric structure would 

 become even more troublesome. 



Significance of Eye-Photophore Similarities 

 Returning, now, to the conclusion that eye-like photophore struc- 

 ture has a more than fortuitous significance, at least two possibilities 

 suggest themselves. The first is that these structures are actually de- 

 rived from eyes. The second is that there is some physical requirement 

 of light-emission that demands a pigment cup-photocyte-lens-cornea 

 sequential structure. Clearly these questions can be "answered," if 

 at all, only in terms of possibilities or probabilities. Nevertheless, they 

 may have some value in suggesting desirable experimental approaches 

 to what I believe is a valid and interesting problem. 



In relation, first, to the possibility that eye-like photophores are 

 actually made-over eyes, mention has already been made of the fact 

 that these organs are often in most un-eyelike locations. There are 

 also such trifling difficulties as the necessities for assuming a conver- 

 sion of photoreceptor pigments to photoemissive compounds, and 

 sensory nerves to motor nerves. However, from the ontogenetic stand- 

 point the possibility is at least not excluded, because it seems to ^e 

 true that in spite of the enormous variety of structure and situation 

 among photophores, all those that have an eye-like structure are, like 



