PHYSIOLOGY OF SYNAPTULA HYDRIFORMIS 361 



could result from photic stimuli alone. It was found that if 

 a similar spot of light was thrown on, a thermometer placed in 

 the same situation, there was an immediate rise from 27 

 to 34C. A column of sea-water 20 cm. high was there- 

 fore not an efficient heat screen. Nevertheless, other experi- 

 ments had shown that synaptulas did not respond to a sudden 

 change of temperature from 27 to 34C. It was thought best, 

 however, to eliminate any possible stimulation from heat rays. 

 Accordingly the animals were placed in a small dish within a 

 box whose sides were painted black. Over a hole in the top 

 of the box was placed a glass tank containing sea-water to the 

 depth of 27 cm. The sunlight came through the water, 

 passed down into the box, and was concentrated on the 

 synaptulas by means of small hand lenses of 2 and 2.5 cm. 

 focal length. When this very small spot of light was allowed 

 to fall on a person's finger, he was unable to perceive any rise 

 in temperature. If the light was thrown on a thermometer, 

 no change could be noted. The results obtained under these 

 conditions are given in table 4. 



TABLE 4 

 The effect of local stimulation by light 



REGION STIMULATED 



RESPONSE 



Base of tentacles 



Body anterior to middle 

 Body posterior to middle 

 Posterior end 



Draws tentacles in. Shortens whole body. 



Local constriction 

 Local constriction only 

 Local constriction only 

 Local constriction. Shortens whole body. 



Bends posterior end away 



All parts of the body of Synaptula respond to local stimula- 

 tion by light of the intensity used in these experiments. The 

 middle portion of the body gives less vigorous response than 

 either the head or the posterior end. The order of sensitivity 

 of parts is, therefore, the same as that for other holothurians 

 (Crozier, '14 a, 14 b; cf. also Parker, '08). 



Headless animals were tested in the same way and were 

 found to give reactions similar to those of entire animals. The 



