36 



THE EYE IN EVOLUTION 



Amoeba 



Paramcecium 



Dendroccelum 



(ciliated on 



ventral surface) 



square, a pseudopod on entering the square will stop for a moment, 

 then protoplasmic flow will commence in the reverse direction, the 

 pseudopod being finally withdrawn from the area. After repeated 

 experiences of trial-and -error, pseudopodia appear on the opposite side 

 of the animal and its whole movement is reversed (Fig. 13). 



Before the response occurs there is a latent period which varies 

 with the intensity of the light ^ ; and if the stimulus be intensified by 

 the use of ultra-violet light, a single stimulus may be sufficient to 

 reverse the direction of locomotion at once. It is also interesting that 

 modifications in behaviour due to experience occur even in organisms 

 so lowly as the amoeba, for the time-reactions of the response are 

 accelerated as the number of consecutive tests is increased, so that the 

 animal becomes habituated to the stimulus (Mast and Pusch, 1924 ; 

 Grindley, 1937). 



As would be expected in this lowly organism, the receptor mechanism is 

 undifferentiated and the response is primitive ; measurements of the elasticity 

 of the plasmagel indicate that the change of movement is due to the gelating 

 effect of radiation on the relatively flviid protoplasm ^ so that flow and the 

 formation of pseudopodia are inhibited on the more highly illuminated parts but 

 can occur readily in those parts of the organism on which the illumination is 

 dim (Mast, 1932). The intimate natvire of the mechanism whereby these changes 

 are brought about is not known. It is noteworthy, however, that similar changes 

 follow mechanical stimulation, and Folger (1926-27) concluded that since light 

 and mechanical agitation produce the same changes and since the two are 

 additive in the sense that the one stimulus can reinforce the other when both 

 are subliminal, the response to the former is perhaps not specifically photo- 

 chemical but of an even more primitive nature. It is also to be remembered 

 that in some cases minute thermal increments are more effective than illumination, 

 so that resjjonses superficially accepted as photokinetic may in fact result from 

 differential heating (differences as small as 0-0005° C are effective in the slime- 

 mould, Dictyosteliurn discoideum, Bonner et al., 1950). 



More mobile Protozoa appear to react with greater effect. Thus 

 ciliated species such as Paramoecium swim about haphazardly but if 

 they approach a noxious stimulus (light, heat, acids, etc.) they back 

 and turn and start off in a different direction, a process which is repeated 

 until, leaving the stimulus behind, they can swim freely forward.^ A 

 reaction which appears more complex is exemplified by the turbellarian 

 flat-worm, Dendrocoelujn (Ullyott, 1936) (Fig. 14). This ciliated 

 flat -worm never travels far in a straight line even if its environment is 

 uniform, but if the intensity of light is increased, although its velocity 

 remains unaltered, the changes in direction occur more frequently, a 



' Pelomyxa — Wilber and Franklin (1947). 



^ That the amoeboid movements of pseudopodia were due essentially to a gel-sol 

 transformation in which the propulsive force is derived from the contractility of the 

 elastic plasmagel was suggested by Wallich in 1863 and the theory was confirmed by 

 Hymaii (1917), Pantin (1924-26) and Mast (1926-31). 



3 Ehrenberg (1838), Jennings (1906), Mast (1911), Rose (1929), and others. 



