48 



THE EYE IN EVOLUTION 



Euglena 



longitudinally by means of cilia, maggots by muscular contraction as 

 they crawl. 



The Flagellates, protozoans which swim by means of a flagellum 

 much after the manner of a gondolier, in reverse, with his single oar, 

 are frequently photosensitive. Some of them retain a primitive photo- 

 kinetic response whereby they become inactive in low illumination and 

 resume activity if the light is increased. This simple kinetic response 

 determining general activity is, however, supplemented by a shock- 

 reaction which determines orientation ; for this purpose they have 

 evolved a sensitized area specially modified for the reception of the 

 stimulus. In a homogeneous environment they take a direct course 

 undergoing continuous rotation on a longitudinal axis as they are 

 propelled by the flagellum ; to variations of the intensity of light they 

 respond by abrupt changes in the rate and direction of movement 

 either towards or away from the light. Once orientated they are not 

 held on a direct course by the continuing action of light, but if they 

 diverge, the orientating stimulus changes and immediately recalls them 

 automatically. The automaticity of the response is seen if the field 

 contains two beams of light crossing at an angle, in which case these 

 organisms orientate themselves and proceed in a direction between the 

 two beams determined by their relative intensities and angles of 

 incidence (Buder, 1917 ; Mast and Johnson, 1932). Their photic 

 responses have been studied most fully in the typical species, Euglena, 

 a transparent green Protozoon photo -positive in weak, photo -negative 

 in strong light. ^ 



Euglena viridis, the flagellate infusorian which commonly forms the green 

 scvim on stagnant fresh water, has a photosensitive " eye-spot " or " stigma " ^ 

 situated in the concavity of a pigmented shield ^ in close association with the 

 root of the flagellum ; the arrangement is such that when the surface of the eye- 

 spot is illuminated the photosensitive substance at the base of the flagellum is 

 thrown into the shadow (Fig. 80). It follows that rotation of the transparent 

 organism on its longitudinal axis produces an alternate shading and exposure 

 of this substance unless it is orientated so as to proceed directly towards or 

 away from a light (Fig. 21). If the direction of the rays is changed through 

 90° to illuminate the organism laterally, no reaction occurs until the rotation 

 brings the eye -spot to face the light thus throwing the photosensitive area into 

 the shade ; thereupon the organism suddenly bends away from the light, and, 

 continviing rotation thus, gradually straightens, a response which is repeated on 

 each rotation so that it is soon proceeding again directly away from the new 

 direction of the light. Subsequent rotation in this position no longer produces 

 changes in the intensity falling upon the two surfaces and the organism therefore 

 proceeds uninterruptedly in this direction. 



1 Verworn (1889), Jennings (1904), Mast (1911-3P,), Bconcroft (1913), Buder (1917), 

 Mast and Gover (1922), Mast and Johnson (1932), and others. 

 ^ p. 126. 

 ' The pigment is astaxanthin, p. 120. 



