50 



THE EYE IN EVOLUTION 



6 



5 V 



^^-. 



N 



111 



M 



Fig. 22. — Klinotaxis in Stentor cceruleus. 



In 1 and 2 the organism is seen swimming away from the hght shining 

 from behind it (indicated by the lower arrows, M). As it swims it rotates so 

 that the oral side (o) and the aboral side (a) are equallj^ stimulated. At 3 the 

 original light is turned off and a lateral light (indicated by the side arrows, N) 

 is turned on. As soon as the oral side faces the light the organism turns rapidly 

 away to position 4 and continues in this sense until, at 6, the oral side is 

 approximately equally exposed to light in all positions on the spiral course 

 (after Mast, 1911). 



^^^axnuP 



Maggot of Musca 



Maggot of Calli- 

 phora 



stroke of the flagella into a backward sweep, the whole number beating in unison 

 and thus orientating the colony in the required direction (Mast, 1906-27 ; Mast 

 and Johnson, 1932). 



Crawling organisms such as the maggots of flies (the house-fly, 

 Musca domestica, the bhiebottle, Callij^Jiora erythrocephala, etc.) were 

 among the first organisms to be investigated in this way.^ Their 

 phototactic response is somewhat different from that of swimming 

 Flagellates or Ciliates. Although the photosensitive structures are ex- 

 ceedingly primitive, the anterior end of the larva is negatively respon- 

 sive to light. When crawling it raises its head in the air and alternately 

 deviates to either side as if in exploratory movements ; on lateral 

 illumination, the head is swung violently away from the light, a reaction 

 which is repeated, turning the animal round until the head is equally 

 illuminated at two successive deviations, whereupon it crawls directly 



1 Pouchet (1872), Holmes (1905), Loeb (1905-18), Mast (1911), Herms (1911), 

 Patten (1914-16), Ellsworth (1933), Welsh (1937). 



