JOHN BUCK 763 



eyes, located on the surface of the body and are presumably derived, 

 like eyes, from the ectoderm. There is also the curious fact that 

 photophores (though not always the most eye-like ones) seem to 

 have a much more than random tendency to occur near, or even 

 partly in, eyes, as might be expected if the two structures are derived 

 from common primordia.^ In so far as Crustacea are concerned, the 

 above-mentioned similarity between photocyte layer and retina could 

 also favor the idea of connnon origin, because of the phylum-limited 

 nature of this type of retinal structure, but the patent lack of similarity 

 between retina and photocyte mass in the cephalopod molluscs 

 weakens the case. 



It is clearly risky to argue from assumed function, but two other 

 structural features of eye-like photophores seem to me more easily 

 understandable as relics of eye structure than as physical necessities for 

 light-emission. These are the "refractor" or "rod bundle" and the pig- 

 ment mantle. Whether the photophore produces a focused sub- 

 marine beam (which I find a little hard to visualize) or diffuse il- 

 lumination covering a wide angle,^ which would appear more useful 

 for the possible functions that the organ might serve (see below) , 

 an apparently collimating device in addition to reflector, lens, and 

 cornea seems superfluous. As to the pigment, it can hardly be argued, 

 for example, that it is necessary to prevent the creature's light from 

 injuring its own tissues, particularly when a reflector is present also. 

 Furthermore, the light is of very low intensity and has a reasonably 

 dispersed source, and many photophores, notably those buried com- 

 pletely in tissue, lack pigment entirely. Likewise, it seems not a strong 

 argument that the pigment is necessary to prevent the light pro- 

 duced from entering the creature's own eyes, since the eyes are well 

 shielded by their own pigment, and photophores are often far distant. 



The second possible "explanation" of eye-like photophore struc- 

 ture — that it is demanded by some physical requirement of light- 

 emission — is easily disposed of. Even if one excludes from the vast 

 array of photogenic organs all those depending on symbiotic bacteria, 



* Practically all writers on luminosity in cephalopods (e.g., 1, 6) have described 

 the colored photophores found on the squid eyeball. Photophores imbedded in 

 the margin of the crustacean compound eye are figured by Chun (5), Kemp (20), 

 and Dennell (9), among others. 



^ Hayashi (15) has in fact described the light of the squid Watasenia as spread- 

 ing out in all directions, and Trojan (28) has diagrammed the course of light 

 through the dioptric system of the Sergestes photophore in such a way that it 

 would also give polydirectional light (though it is not clear that he actually saw 

 this occur), but neither observation is critical enough for our purpose. 



