52 



THE EYE IN EVOLUTION 



TROPOTAXIS 



In tropotaxis at least two symmetrical receptor organs are neces- 

 sary, and instead of relying on successive exposures of a single receptor 



1 1 1 J 



Fig. 24. — The Tropic Response of Larva of Aresicola. 



A. The head of the larva with two symmetrical eyes. 



B. The path of movement of the larva : in 1 to 4 the light remains 

 stationary ; in 5 to 8 it is placed at right angles (after Mast). 



by trial movements, the animal orientates itself by the simultaneous 

 comparison of the intensity of stimulation on the tivo sides. In the simple 



r /5 

 \3 



Fig. 25. — Negative 

 Tropotaxis. 



The path of the flour- 

 moth larva, Ephestia, 

 starting from the small 

 circle with a light shown 

 as indicated by the arrow, 

 1. Each successive num- 

 ber indicates the position 

 of a new light turned on 

 when the animal reached 

 the corresponding point 

 on its track ; its direction 

 changed in a straight line 

 directly from the light 

 (after I3randt). 



forms inequality of stimulation leads to orienta- 

 tion in the required direction by a reciprocal 

 coordination of the muscles of either side of the 

 animal controlled by the nervous system : if 

 there is an excess of stimulation on one side, a 

 turning movement occurs ; if equality, the 

 stimuli cancel each other out and the animal 

 progresses straight forwards ; and if it subse- 

 quently strays from its path a renewed in- 

 equaUty corrects the deviation. It follows that 

 if two sources of light appear simultaneously 

 the animal orientates itself directly between 

 them in proportion to their relative intensities. 



The larvae of some marine worms provide the 

 most simple type of this reaction ; they swim by the 

 activity of cilia but orientation is the result of 

 muscular contraction. Of these, the larvae of the 

 polychsete worm, Arenicola, have been most inten- 

 sively studied (Mast, 1911 ; Garrey, 1918). These are 

 minute creatures with two eyes anteriorly and a band 

 of cilia at either end ; as they swim they rotate longi- 



