362 J. M. D. OLMSTED 



severed heads, however, behaved somewhat differently. When 

 the spot of light from above was thrown on them, they at once 

 turned and moved off at approximately a right angle to the 

 direction in which they were first moving. Only once did a 

 head attempt to draw in its tentacles; that was after the spot 

 of light was kept focussed on it as it moved about. At least 

 10 seconds of constant stimulation was necessary to produce 

 this reaction, whereas in entire animals this response was prac- 

 tically instantaneous. 



The results of these experiments make it seem doubtful if 

 the 'eye-spots' are 'light-detecting' organs. For, first, the 

 whole body is sensitive to light, and, secondly, parts of the 

 body without eyes are able to orient away from a source of 

 light. Nevertheless, one must take into account the fact that 

 in headless, and therefore eyeless, specimens orientation did 

 not occur in light of less intensity than direct sunlight, while 

 the heads of the same animals, which contained the eyes, did 

 orient under the same conditions. This is simply further in- 

 dication that the head end is more sensitive than other parts of 

 the body, and in no way does it prove that the presence of the 

 eye-spots is responsible for the greater sensitivity. Parker 

 ('08) found that the anterior end of Amphioxus, which contains 

 a so-called eye-spot, was more sensitive to light than either the 

 middle or posterior part of the body. Yet he showed that this 

 eye-spot was not a photoreceptor, because light focussed upon 

 it elicited no response from the animal. The true photore- 

 ceptors in Amphioxus were certain organs distributed along 

 (he nerve cord, more of them being found at the anterior end, 

 the region of greater sensitivity. This has been confirmed 

 and further demonstrated by Crozier ('17). Similarly in Synap- 

 tula, the whole surface of the body is sensitive to light, the 

 anterior end being more so than the other parts, perhaps in 

 spite of, rather than because of, the eye-spots. 



4. The effect of heat 



a. Historical. Few investigations have been made upon the 

 reaction of holothurians to heat. Crozier ('14 a) found that 



