114 



THE EYE IN EVOLUTION 



Dytiscus 



Myxine 



A nodonta 



The mechanism of this dermal sensitivity is conjectural. The 

 reaction may be initiated by photosensitive pigments and, although in 

 most cases such have not been identified, they could be present in very 

 small amounts (von Uexkiill, 1897). On the other hand, it is conceiv- 

 able that nerve elements lying subdermally may be directly stimulated, 

 a view for which Millott (1954-57) produced firm evidence in the case 

 of the sea-urchin, Diadema. Such a reaction would correspond 

 to the activity of the organelle of the apolar hght-sensitive cells of 

 worms, the sixth abdominal ganglion of crayfish and lobsters,^ and the 

 nerve elements in the diencephalon of lampreys, minnows and ducks. ^ 

 Again, Bohn (1940) and Viaud (1948) looked upon the reaction as a 

 common property of protoplasm depending on " electrochemical 

 polarization", a property readily evident in lower forms but often 

 neutralized or masked by more potent reactions in higher forms. 



Such a dermal light-sensitivity (the dermatopsia of Graber, 1884) is of 

 wide distribution occurring in members of almost all phyla. ^ While it is usually 

 diffuse it may be particularly well developed in certain situations wherein its 

 biological utility is greatest, often the fore-pai't of the animal or in such situations 

 as the region of the spiracles of the abdomen of the larv.'B of the water-beetles, 

 Acilius and Dytiscus (Schone, 1951). Such a sensitivity is particularly marked 

 and widesj^read among Echinoderms (Cuenot, 1891) ; it occurs in some Molluscs, 

 Turbellarians and Annelids, as well as in some Insects, in Cyclostomes [Myxine 

 glutinosa, Newth and Ross, 1955), in eyeless cave-fish (Thines and Kahling, 1957) 

 and in blinded cat-fish. The response to dermal sensitivity is, of course, a photo- 

 kinesis which may be either jjositive or negative. Thus the eyeless mussel, 

 Anodonta, reacts to a passing shadow (Knoop, 1954 ; Braun and Faust, 1954), 

 blind cavernicolous beetles (Anophthalmns) respond to the light of a candle 

 (Marchal, 1910), and after complete blinding some insects, such as cockroaches, 

 will settle preferentially in the dark,^ a reaction which may persist even after 

 decapitation,^ while others are attracted to light.* 



It is to be noted that the dermal response to light need not be of the 

 same type as the ocular response ; the two may, indeed, be mutually exclusive. 

 Thus it will be remembered ' that the flat-worm, Planaria lugubris, is positively 

 photokinetic so far as the dermal response is concerned while it orientates itself 

 by a negative phototaxis through the eyes (Viaud and Medioni, 1949). Again, 

 the receptors in the skin and the eyes may show different sensitivities. Thus 

 Viaud (1948) found that in some organisms the maximum response of the dermal 

 mechanism was elicited by wave-lengths at the short end of the visible spectrum 

 (the water-flea, Daphnia ; the rotifer, Branchionus) while the eyes responded 

 preferentially to wave-lengths about the middle of the spectrum. A com- 

 bination of the two mechanisms in the same organism may thvis involve two 

 maxima in the response (as in the fruit-fly, Drosophila). 



Daphnia 



^ p. 115. 2 p_ 537 



3 For reviews, see Willem (1891), Dubois (1892), Nagel (1896). 

 Viaud ( 1948) whose views have already been discussed on p. 31. 



4 i.'A/^eito— Graber (1883) ; PeW;j7aneta— Brecher ( 1929). 



^ The larvsG of the meal-worm, Tenebrio — Tucoleseo (1933). 



M erpillars— Lammert (1925), Suffert (1932), Oehmig (1939). 



' p. HtJ, 



See especially 



