The Comatulids of Torres Strait. 117 



more briefly, the response to heat. The tank previously mentioned 

 was used for these experiments, and while the results were not entirely 

 consistent, they were sufficiently so to make them of some value. An 

 alcohol stove placed under one end of the tank made it possible to 

 raise the water just over the flame several degrees without appreciably 

 affecting the temperature of the other end for nearly an hour. 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. Comatulids placed in the tank at 

 the unheated end began at once to move towards 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 

 withdrawal 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 individual 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 1| hours the water in the tank was 

 raised to a minimum of about 29 and the comatulids were all at rest 

 or moving very languidly. It was interesting to note, however, that 

 transfer from this warm water to the sea-water at ordinary tempera- 

 ture (26.6) 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. As a result of these observations, it is evident 

 that comatulids are sensitive to comparatively slight changes of tem- 

 perature 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 withdraw at once, if possible, from 31. Evidently, then, a 

 fairly uniform temperature is an essential factor in determining 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 mid-day, 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 on Maer has such an abundant 

 comatulid population. The abundant food supply in that region is 

 thus not the only factor in the case. 



