MOVEMENT TOWARD LIGHT IN COELENTERATES i6l 



of light, some deflect to the right, others to the left, so that 

 the group gradually becomes wider and wider. When 

 exposed to light from two sources they may travel toward 

 any point between, as stated under Euglena. 



They are positive in strong as well as in weak light, but 

 if the intensity is very high, as, e.g., direct sunlight, or 

 quite low, they do not orient so accurately as they do in 

 light of moderate intensity, and the lateral movements of 

 the anterior end are more pronounced. In general the more 

 strongly positive the planulae are, the more accurately they 

 orient and the less they swing the anterior end from side 

 to side. From time to time the anterior end also becomes 

 flatter and broader. The lateral movements of the anterior 

 end, as well as the process of becoming broader, are due to 

 internal contractions. If the ray direction is but slightly 

 changed after the planulae are oriented, they do not turn 

 directly toward the source of light in its new position, but 

 merely swing the anterior end a little farther toward it 

 each time. In the meantime the body gradually turns so 

 as to become oriented again. If however the direction of 

 the rays is changed to such an extent that the sides of the 

 organism become fully exposed, they with very few excep- 

 tions appear to turn toward the light at once. In this 

 process they swing the anterior end laterally until it nearly 

 if not quite faces the source of light. It is thus frequently 

 bent at right angles to the posterior end. The anterior 

 end often swings back after turning but never so far as it 

 was before. The lateral turning is a slow steady movement 

 due, no doubt, to contraction of the tissue in the planulae 

 and not to the action of the cilia, for it is much more rapid 

 than the forward movement, which is entirely due to the 

 action of the cilia, and moreover no currents indicating 

 unequal ciliary action on opposite sides could be detected. 



There Is no definite reaction if the intensity is suddenly 

 decreased or increased, nothing similar to an avoiding reac- 

 tion or a shock movement. I have seen the planulae pass 

 from darkness into strong light, consisting of rays perpen- 



