BEHAVIOR OF HOLOTHURIANS IN BALANCED ILLUMINATION 511 







either side could not in any position be made equal to that incident 

 upon the other. It is assumed, of course, that the longitudinal axis of 

 the animal remains straight when orientation is completed. 



Thus an animal of this kind might be clearly phototropic according 

 to other criteria, and yet fail to satisfy the requirement of orientation 

 under bilateral illumination which Loeb's contention demands. The 

 existence of such organisms, however, would add further support to the 

 assumption made by Patten, namely, that the exhibition of a definite 

 path of locomotion under these conditions depends upon the final equal- 

 ity of illumination on each of the two photosensitive surfaces. 



III. Certain pedate holothurians have been found to be different from 

 most other echinoderms, in that they exhibit a pronounced bilaterality 

 in their structure, coupled with a fixed direction of progression (4), (5), 

 (6). Typical tropistic behavior is thus made possible. It has been 

 shown that some of these holothurians (Holothuria surinamensis, H. 

 captiva) are photo-negative, that they are sensitive to light over their 

 whole surface, that they give w y ell defined reactions to shading, and 

 are non-reactive to a sudden increase in light intensity, yet orient 

 away from the light with conspicuous precision. It is impossible to 

 conceive, as I have already pointed out (4), that these animals are ori- 

 ented by light in any way other than through the direct action of illumi- 

 nation upon their integument. 



One of the holothurians referred to, H. captiva, is the most outstand- 

 ingly bilateral of all the species which I have studied. Its lateral sur- 

 faces are so nearly straight and parallel that, to all intents and purposes, 

 they may be regarded as strictly so; that is, longitudinal elements of 

 its surface are parallel. I have previously described the accuracy 

 with which it orients away from the light. Experiments were subse- 

 quently made to determine the behavior of this species when illumi- 

 nated from opposite sides. 



Inasmuch as the whole surface of H. captiva is sensitive to light, the 

 higher sensitivity of the oral end should not greatly obscure the inter- 

 pretation of such experiments. When horizontal light is employed, the 

 sides of the animal may be regarded as equivalent to two photosensitive 

 areas. This particular species of holothurian, fortunately, tends to 

 preserve a straight position of the body. 



In my present study upon this animal, the tests were made in a dark 

 room, the animals being placed in a flat-sided aquarium with their long 

 axes perpendicular to the line connecting the two sources of light. 

 The lights used were two tungsten incandescent filaments, placed at 



