410 



Comparative Animal Physiology 



a wave length of 474 /x, in a spectrum uncorrected for energy.^*'" Hecht^^- ^-^ 

 measured the spectral sensitivity of the photic response of the clams Mya 

 and Pholas in relation to broad regions of the spectrum. He found a maxi- 

 mum for Mya at 490 /i, and for Pholas at 555 /a, with perhaps another 

 maximum in the ultraviolet. The spectral sensitivity of the squid retina sug- 

 gests that the photosensitive pigment involved is similar to the vertebrate 

 pigment, rhodopsin. 



Bliss,^^' ^^' ^^ isolated a pigment from squid retinae which resembles the 

 vertebrate rhodopsin (Fig. 118). The absorption maximum is 495 ix, and 

 its absorption spectrum agrees well with the absorption spectrum of visual 

 purple and with the spectral sensitivity of the photoreceptor. There is, how- 



500 

 W»VP-lrn«th (m/i) 



Fig. 118. Action spectrum of the dark-adapted human eye (after Ludvigh,"") com- 

 pared with the spectral absorption curve of visual purple. From Granit." 



ever, one difficulty in assigning to this pigment a principal role in the visual 

 process; the squid pigment is light stable. Bleaching in the presence of light 

 occurs only when the extracted pigment is sensitized by a dilute denaturant, 

 such as formalin. The bleaching product is identical with visual yellow of 

 the rhodopsin cycle (page 411) and the carotenoid has been identified as 

 retinene]. The spectral sensitivity of several arthropod eyes,"*"- ^^' ""• '""* de- 

 termined by electrical (see page 422) and behavior methods,^"'' indicates ab- 

 sorption maxima in the blue portion of the visible spectrum, and a second 

 maximum in the ultraviolet, wave length 385 /x, for Drosophila and the 

 bee.'"' ^' High concentrations of vitamin A have been isolated from the eyes 

 of several marine Crustacea, the green and fiddler crabs and lobster, and 

 from the eyes of the fresh-water crustacean, Cauiharns viriJis. The eyes of 

 the latter also contain retinenci, in addition to astaxanthin. 



In the vertebrate eye the role of carotenoids in photoreception has been 

 clearly set forth. The eyes of marine fishes, frogs, turtles, birds, and mammals 

 contain a rose-colored photolabilc pigment associated with rod function in 

 the duplex retina. This pigment has in the past been called visual purple. 



