120 



THE EYE IN EVOLUTION 



Flagellates {Eucjlena, etc.) has been identified as astaxanthin (di- 

 hydroxy di-keto /3-carotene, C40H52O4) (Tischer, 1936-38; Kiihn et al., 

 1939) ; this pigment is found only in animal tissues, is produced by 

 the modification of ingested plant carotenoids, and, depending on its 

 chemical nature, may range in colour from blue to red with varying 

 characteristics of absorption. The pigments associated with the 

 photoreceptors of the lower Invertebrates have not been fully investi- 

 gated, but the available evidence indicates that the phototropic 

 responses of the polyps of Coelenterates ^ and the siphons of clams, ^ as 

 well as the phototactic activity of worms, ^ are also mediated by 

 pigments of the astaxanthin type (Fig. 78). 



On the other hand, when image -forming eyes are reached in 



Z 



o 

 ^ 



X 



UJ 



10 

 08 

 6 

 04- 

 0-2 

 



■^^^'si^ — 



100 



80 

 60 

 40 

 20 

 







400 



500 



600 



Wave/enohh -mu 



Fig. 78. — Spectral Sensitivity for Photo-orientation of the Green 



Flagellate. 



Absorption spectrum (extinction) for astaxanthin dissolved in hexane ; 

 continuous line (after Wald). Relative spectral sensitivity of Euglena viridis ; 

 broken line (after Mast). 



Molluscs and Arthropods, the power has been gained to degrade 

 vegetable carotenoids into the vitamin A system. Thus among 

 Molluscs, the retina of the squid, Lolicjo pealii, has been found to have 

 considerable quantities of retinene^ and vitamin A^ which, combined 

 with a specific protein, produces a pigment with absorptive charac- 

 teristics resembling those of rhodopsin (Wald, 1941 ; Bliss, 1943-48 ; 

 St. George and Wald, 1949 ; Hubbard and St. George, 1956). It 

 would appear that in the squid this reddish photopigment is a non- 

 photosensitive type of rhodopsin, for which reason it was distinguished 

 as cephdiopsin b}^ its discoverer (Bliss, 1948). 



^ Hydioids of Sertularia and Eudendriuw , maximum absorption 474 m/u, Loeb 

 and Wasteiii -s (1915). 



^ Mya. iximum absorption 490 ni/it, PJiolas, maximum absorption 555 m/x, 

 Hecht (191; 



* The eji i irm, Lumhricus lerrestris, and the larva? of the marine worm, Arenicola, 



maximum abs. ion 483 m/i. Mast (1917). 



