512 W. J. CROZIKR 



various distances from the holothurian in different experiments. In 

 some cases the animal was induced to begin crawling in a direction per- 

 pendicular to that of the light beams before the latter were turned on. 

 The intensity of the lights was varied by using filaments of different 

 wattage (25 to 100 watts). Since the characteristic features of the re- 

 sults soon became evident in these experiments, no great refinement 

 was necessary in the physical precision of the light adjustments. 



IV. In no case did Holothuria captiva assume a path of locomotion 

 definitely related to the intensities of the acting lights. In no case 

 was it constrained to move, after the fashion of the blowfly larva, along 

 a path inclined to the direction of the beams. A number of animals 

 were found to move first toward one light, then toward the other; this 

 was the case when the lights were of nearly equal intensity. Many 

 individuals circled in an aimless, indeterminate way (as when illumi- 

 nated from above), while others became curled into the shape of the 

 letter S, the oral end, and frequently the cloacal end as well, being bent 

 away from the stronger light. 



When the intensity of the stronger illumination was made eight or 

 nine times that of the weaker one, the holothurians almost invariably 

 oriented precisely and away from the stronger light. But when they 

 had moved somewhat toward the weaker light, their movements be- 

 came quite irregular. 



To meet the criticism that possibly the incandescent filaments as 

 sources of light were. too small as compared with the size of the ani- 

 mals, the latter being 6 to 8 cm. in length, experiments were tried with 

 sunlight reflected from two mirrors, each measuring 15 by 20 cm. The 

 holothurians were in a dark chamber, an opening on either side (10 by 

 8 cm.) allowing the sunlight reflected from the mirrors to enter the box 

 from opposite directions. One or several lightly smoked glasses were 

 then interposed between one mirror and the corresponding opening in 

 the dark chamber. The behavior of the holothurians under these con- 

 ditions was qualitatively identical with that in the trials with electric 

 lights. Hence I conclude that H. captiva does not maintain a pre- 

 dictable path of locomotion, under the influence of bilateral illumination. 



V. The validity with which this result may be applied to the criti- 

 cism of photic stimulation theories depends largely on the share taken 

 by the general surface of the animal in photoreception. That this share 

 is a considerable one, is shown by the following: 



a. Excised pieces of the "dorsal" integument (including skin and 

 muscle layers) which have been cut from the midbody surface of H. 



