692 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The sea water brought from the reef and with which the experiments began 

 had a temperature of 26.6 C. The area directly over the stove soon became about 

 a degree warmer than this, while immediately above the flame it rose to 31.7 C. 



Comatulids placed in the tank at the unheated end began at once to move 

 toward the other end, the whole tank being uniformly illuminated. The passage 

 into the slightly warmer water was not indicated in any way, but whenever the 

 arms extended into the area above 30 a more or less prompt response and with- 

 drawal followed. 



This was shown by all the species tested, but was most noticeable in adult 

 Comatula purpurea, and consequently that species was chiefly used. If an indi- 

 vidual were placed in the hot area it immediately moved out, but it did not seek 

 the coolest part of the tank. 



At the end of an hour and a half the water in the tank was raised to a 

 minimum of about 29, and the comatulids were all at rest or moving very lan- 

 guidly. It was interesting to note, however, that transfer from this warm water 

 to the sea water at ordinary temperature (26.6 C.) acted as an immediate stimulus, 

 and locomotion was at once renewed. The reverse was also true, comatulids from 

 the normal sea water responding at once with active movements when dropped 

 into the warm water. In the latter case, however, the movements very quickly 

 ceased. 



Doctor Clark concludes from these observations that it is evident comatulids 

 are sensitive to comparatively slight changes of temperature, and that for those 

 living at Maer 26 to 27 C. is about the optimum. It is not probable that the 

 sea water on the reefs at the Murray Islands ever gets far below that temperature, 

 but it is often much higher, at least on the reef flat itself. It is therefore of great 

 interest to find that at 29 these comatulids become inactive, and that they with- 

 draw at once, if possible, from 31. 



Evidently, then, a fairly uniform temperature is an essential factor in deter- 

 mining their distribution, and this accounts for their absence from the greater 

 part of the reef flat at Maer, for the water on some parts of this flat, if low tide 

 is near midday, rises far above 31. It seems probable, too, that the more uniform 

 temperature and the absence at all times of high temperatures in the channel 

 between Maer and Dauer are added reasons why the southwestern reef of Maer 

 has such an abundant comatulid population. 



Observations at Tobago. While it was not feasible to produce really low 

 temperatures, water in the tank fell to 22 C. during the night. At this temper- 

 ature Tropiometra picta showed no signs of discomfort or stimulation. When the 

 water was gradually raised to 33 or 34 responses to mechanical stimuli were 

 normal. At 35. 5 two of the four specimens tested were quite unresponsive, but 

 the other two gave normal responses by arm movements. One continued to respond 

 at 36. 5 but at 37.5 was inert. After a few moments at that temperature the 

 four specimens were placed in normal sea water at 28, but none recovered. 



Individuals taken from water at 28 and placed in that at 33 continued to give 

 responses with the arms. At 34. however, the arms were quiescent, but the pin- 



