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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 Maér, 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 Maér, 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 Maér and Dauer are added 
reasons why the southwestern reef on Maér has such an abundant 
comatulid population. The abundant food supply in that region is 
thus not the only factor in the case. 
