BENT LEY (.LASS 90!) 



niuin absorption at ;■)()() ni^, like rhodojjsin, although it probably con- 

 tains retincnco, like the visual pigment ol most other freshwater fish. 

 The achromatic ganglion cell type that gives both "on" and "off" 

 responses throughout the spectrum fits nicely the characteristics of a 

 pigment absorbing maximally at 600 \\\fx. The long wavelength sys- 

 tem has maximum absorption at about 650 xxifj., but docs not give 

 a good fit to any particular hypothetical curve for a single visual 

 j)igment. No visual pigments absorbing at 600 niyu, or 650 niyu, have 

 been isolated to date, but the carp has j^rovided microspectrophoto- 

 metric evidence of a pigment with an absorption maxinunn at 610 

 niytt (Hanaoka and Fujimoto) . These studies, just beginning, offer 

 promise of many interesting things to come. 



The Relation of Photic Orga}is to Eyes 



John Buck points out the existence of "a provocative structural 

 similarity betAveen certain eyes and certain photophores," a resem- 

 blance which it hardly seems possible to attribute to mere change 

 or to any type of evolutionary convergence. Many organs serving 

 in deep-sea fishes, squids, and shrimps for the production of light 

 not only possess an external transparent layer like a cornea, a lens, 

 photic tissue in the approximate position of a retina, a reflector 

 layer (tapetum) , a pigment cup, and a supplying nerve, but further 

 resemble the vertebrate, molluscan, or arthropod eye in having a 

 striated "refractor" or "rod-mass" in front of the photogenic tissue; 

 or in having rod-shaped photocytes arranged in a layer like the 

 arthropod retina. Three good examples among arthropods may be 

 cited: (a) the thoracic photophores of schizopod shrimps, and the 

 ocelli of pseudoscorpions and pedipalps; (b) the eyestalk and gill 

 chamber photophores of decapod shrimps, and the ocelli of insects; 

 and (c) the eyelike photophores on the bases of the legs of many 

 decapod shrimps, and the crustacean compound eye. 



Photophores are often in very uneyelike locations, yet also often 

 occur adjacent to eyes or even within them, as if they might have been 

 derived from some common type of rudiment. In terms of their 

 light-emitting function, it seems especially hard to account for the 

 presence of the pigment layer, which seems quite unnecessary, or the 

 "refractor" structure, which seems altogether superfluous in the pres- 

 ence of reflector, lens, and cornea. And only when, as Buck points 

 out, they are located in the integument, w^here the need would seem 

 to be least, do photic organs conmionly possess a cornea and lens. 

 It is quite clear, furthermore, that an eyelike structure is not essential 



