RHYTHMIC PULSATION 



337 



of the isolated cloacal ends. Stichopus is much less sensitive 

 to photic irritation than is Holothuria, and no influence of light 

 upon either rate or amplitude of pulsation could be detected. In 

 Holothuria it has been shown with some degree of probability 

 that the green fluorescent integumentary pigment acts as a photo- 

 sensitizer (Crozier, '14); hence a photic effect upon cloacal pulsa- 

 tion would be much more probable in this case. 



Isolated cloacal extremities of Stichopus which had ceased to 

 pulsate regularly in sea water, though otherwise in apparently 

 good condition, were placed in moderately bright sunlight. This 

 did not stimulate to pulsations. "When-such pieces were shaded, 

 the sphincter reacted by closing fairly tightly; the reaction time 

 was about 1.2 seconds at 25.0°. Pieces which were pulsating 

 slowly (130 seconds ± for 10 movements) gave the same reac- 

 tion, the sphincter closing promptly when the shadow was cast 

 during the open phase, but failing to react during a period of 

 closure. Light did not accelerate the rate of movement in nor- 

 mally pulsating cloacal ends. 



The isolated cloacal extremity of H. surinamensis, however, 

 gave the following result: 



Experiment 53.2. Seven specimens of Holothuria surinamensis had 

 the posterior ends (1.5 ems.±) removed. After remaining about 30 

 minutes in diffuse daylight, their pulsation rates were observed (column 

 a). Four of the pieces were then placed in bright sunlight (b'), the other 

 three remaining in the diffuse daylight (6). After 15 minutes the 

 pulsation rates were again determined (6, 6') : 



