Reprinted from THE AMERICAN JOURNAL OF PHYSIOLOGY, Vol. 43, No. 4 



July, 1917 



CONTRIBUTIONS FROM THE BERMUDA BIOLOGICAL STATION 



FOR RESEARCH, NO. 68. 



THE BEHAVIOR OF HOLOTHURIANS IN BALANCED 



ILLUMINATION 



W. J. CROZIER 

 Received for publication May 3, 1917 



I. The behavior of phototropic animals when so situated as to be 

 illuminated from opposite directions gives important information re- 

 garding the nature of the mechanism of Stimulation. It was held by 

 Loeb (1) that a phototropic organism, when placed at the center of the 

 line joining two equal sources of illumination, should move in a direc- 

 tion perpendicular to this line. The accuracy with which this kind of 

 response is obtainable in a suitable organism is demonstrated by Pat- 

 ten's (2) experiments with the negatively phototropic larva of the 

 blowfly. Patten further found that the angular deflection of the larva's 

 path from the perpendicular was so related to the percentage difference 

 in intensity between the two lights, when these intensities were caused 

 to alter in a graded series of ratios, as to produce a smooth curve when 

 these values were plotted on the axis of ordinates and abscissae, respec- 

 tively. On the basis of this graph, and taking into account the blowfly 

 larva's method of locomotion, Patten was able to show (3) that the 

 responses appear to be determined by the presence of two photosensi- 

 tive surfaces inclined to each other at a definite angle. The definite 

 path of progression adopted by the larva under the influence of two op- 

 posed unequal beams may be predicted on the assumption that the ani- 

 mal ceases to deflect from its original locomotor path which in these 

 experiments is a straight line normal to that connecting the sources of 

 light as soon as the luminous intensity on these mutually inclined sur- 

 faces is made equal by the larva's position. 



II. There may be deduced from this principle the corollary that, if 

 the photoreceptive surfaces of an animal are so arranged as to be sen- 

 sibly parallel, no definite position should be assumed by this organism 

 when bilaterally illuminated. For, in such a case, if the balanced lights 

 were equal in intensity, it would be impossible for the animal to place 

 itself in a position which would result in unequal illumination of the two 

 sides; while, if the lights were unequal, the amount of light received by 



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