122 THE EYE IN EVOLUTION 



forms in the vertebrate phylum the vitamin A^ system is again 

 encountered. Wald (1939-56) considered that two specific pigments 

 were concerned — rhodopsin (visual purple) with the vitamin A^ 

 system and porphyropsin (visual violet) with the vitamin A 2 system 

 (Fig. 79). Evidence is rapidly accumulating, however, that the matter 

 is not so simple, for it would appear that each of these does not repre- 

 sent a single specific pigment ; both vitamins Aj and A 2 can exist as a 

 number of isomers some of which combine with suitable proteins to yield 

 photosensitive pigments of distinctive absorptive properties, several of 

 which have already been discovered. Rhodopsin should therefore be 

 interpreted as a generic name for all visual pigments associated with the 

 rods based on vitamin A^, while porphyropsin is best similarly 

 interpreted as embracing several rod -pigments based on vitamin A 2 

 (see Dartnall, 1957). 



The photosensitive pigments so far claimed — although with little 

 substantial evidence — to be present in vertebrate cones — iodopsin 

 associated with the vitamin Aj system and cyanopsin associated 

 with vitamin A 2 — are also related carotenoid-proteins (Wald, 1937-55 ; 

 Bliss, 1946) ^ ; on the other hand, accessory needs in the visual 

 system such as the yellow pigment of the human macula are said to 

 be met by xanthophyll — the intact carotenoid which mediates photo- 

 reception in plants. 



The multiplicity of pigments of these two general types associated with the 

 visual system is becoming increasingly apparent, and odd varieties have been 

 discovered in special circumstances, differing considerably from the main groups. 

 As we have seen, fresh -water fish usually have a pigment of the porphyropsin 

 family, salt-water fish of the rhodopsin family ; euryhaline and migratory 

 fishes which adapt themselves to both fresh and salt water therefore present an 

 interesting problem. Since their spectral absorption curve is intermediate 

 between that of rhodopsin and porphyropsin, Wald (1941) concluded that 

 their retinae contained a mixture of both ; but it has been shown by Munz 

 (1956) that in one at least of these fishes (the mud-sucker, Qillichthys mirahilis) 

 the retina contains a single homogeneous pigment characteristic of the retinene^ 

 type with an absorption maximum intermediate between the two main groups 

 (512m!JL). Again, the gecko (Gekko gekko) has an unusual spectral sensitivity 

 curve, similar to the human scotopic curve but with its maximum displaced 

 20 to SOmji, towards the red end of the spectrum (Denton, 1956). Retinal 

 extracts from the Australian gecko, Phyllurus milii, have shown the presence 

 of an unusual pigment with an absorption maximum at 524m(ji, typical of the 

 retinene^ system but intermediate between the rhodopsin of the rods and the 

 iodopsin of the cones (Crescitelli, 1956). This is interesting in view of the theory 

 that the rods of this nocturnal animal may be transmutations from the cones 

 of ancestr i cUurnal lizards.^ 



Pigments of unknown composition and tinknown function which appear, 



^ In tilt -es of primates three pigments have been detected: chlorolabe (a green- 

 absorbing ]i nt), erythrolabe (red-absorbing), and cyanolabe (blue-absorbing). See 

 Vol. IV, p. 4 



2 p. 252. 



