126 



THE EYE IN EVOLUTION 



viridis, is of the simplest variety (Fig. 80) ; the entire structure is 

 about 5/x in diameter (Franz, 1893). The base of the single flagellum 

 shows a thickening just anterior to its bifid root in the cavity of the 

 reservoir ; it would seem that this is the photoreceptive area and it is 



BLIND SlD£ 



S£EIN0 SiD^ 



Fig. 80. — The Eye-spot of Euglesa viridis. 

 A side view of the anterior end of the flagellate. /. The flagellum with 

 an enlargement, e/, which constitutes the photoreceptive area. The two roots 

 of the flagellum are anchored in the large contractile vacuole {cv.). Opposite 

 the sensitive area is a shield of pigment (e) (after Wager, 1900). 



flanked on one side by a shield of the red carotenoid pigment, astaxan- 

 thin (Engelmann, 1882 ; Wager, 1900 ; Mast, 1911-38 ; Buder, 1917 ; 

 Tischer, 1936-38 ; Kuhn et al., 1939). It is interesting that Ehrenberg 

 (1838), who first described this flagellate, assumed that the pigmented 

 area was light-sensitive and considered that it constituted the most 

 primitive eye in nature and called it the eye-spot 

 (" Augenfleck"). It seems more probable, however, 

 that it serves as an absorbing agent, shielding the 

 flagellar swelling from incident light on one side and 

 allowing it to be exposed on the other, so that as 

 the organism rotates in swimming, the alternating 

 stimulation and shading of the stigma affect the 

 beat of the flagellum and directional phototaxis is 

 attained.^ 



The instability of such an eye-spot is intei'esting. The 

 Pringsheims (1952) found that if Euglena gracilis were 

 grown at temperatures below the optimuni the pig- 

 ment was lost and an apoplastidic race was produced in 

 which the stigma disappeared if the organism were kept 

 in the dark ; once lost, the eye-spot did not reappear. 



In certain Dinoflagellates, organisms which form an 

 important part of the plankton of lakes and the sea, the 

 1 p. 48. 



Fig. 81.— The Eye- 

 spot OF PoUCHETIA. 

 Showing 1 hf large 

 pigmented /- ■•, P, 

 and the lens, '■■ ifter 

 Schiitt). 



