298 W. J. CKOZIER AND LESLIE B. AREY 



b. Change in temperature has an immediate effect upon the 

 rate of the heart beat in Chromodoris. The thin-walled heart 

 lies near the dorsal surface of the animal, immediately anterior 

 to the gill crown. Its pulsations can be plainly seen and rather 

 accurately 'timed' in the intact animal. The rate of contraction 

 of the auricle varies greatly in different individuals under closely 

 similar external conditions — from 7.2 to 22 seconds being re- 

 quired for ten beats at 24° to 25°C. in a number of animals 

 examined. The frequency of the beats is readily controlled by 

 the temperature, as may be seen in the following notes of an 

 experiment with one individual: 



Individual A 



Temperature C . Seconds for tO beats 



25° (Room temperature) 22.0 



30° after 3 minutes immersion 14.4 



after 15 minutes immersion 14. 



after 20 minutes immersion 13 . 6 



after 40 minutes immersion 20 . 5 



25° 20 . 5 



35° after 5 minutes immersion " 11.0 



after 10 minutes immersion 10 . 5 



after 15 minutes immersion 10.0 



In this experiment the pulsation rate is a little more than 

 doubled by a rise of 10° (25° to 35°), but the relation between rate 

 and temperature is a linear one (fig. 7, A), not 'exponential.' 

 Other experiments checked this finding (fig. 7, B), so far as the 

 temperature range 20° to 35° is concerned. At temperatures 

 above 35° the heart beat became faint and irregular, following a 

 preliminary acceleration, and hence difficult to count. Below 

 18° the heart beat could not easily be counted in the intact 

 animal, because of its faintness, but it seemed (see dotted line, 

 fig. 7, B) that the straight-line relation between frequency and 

 temperature was departed from at the lower temperatures. 



c. In the attempt to localize possible areas of special thermal 

 sensitivity, small volumes of warm water were applied from a 

 pipette to different regions of the animal's surface. The an- 

 terior end seemed the most sensitive part, but it was not deli- 

 cately so. A stream of water running under low pressure from 



