300 W. J. CROZIER AND LESLIE B. AREY 



Sunlight concentrated by a lens of 30-cm. focal length gave 

 a very hot spot of light even when screened by 30 cm. of sea- 

 water. This spot was allowed to fall upon different parts of Chro- 

 modoris' surface. Even though to one's hand the heat stimu- 

 lation by this focussed beam was intense, it evoked only local 

 reactions — absolutely restricted to the part stimulated— on all 

 parts except the buccal veil and mouth region. The buccal veil 

 and the tentacles were the most sensitive parts, and they were 

 about equally sensitive, so far as could be judged. The 'rhino- 

 phores' gave merely weak homolateral responses, but when the 

 spot of (light and) heat was allowed to fall on the oral area, 

 the animal bent sharply away. Immediately outside the central 

 part of the focussed beam, when not sharply focuesed, the heat 

 effect was slight, as tested with a thermometer, but the light cone 

 brilliant; this light did not, however, stimulate the integument 

 of the oral region. Therefore the foregoing responses must have 

 been due to heat. 



Experiments were then made in a trough 30 cm. long, contain- 

 ing sea-water and heated at one end. When the water at this 

 end was at 32° to 33°C. and that at the other 25°, a Chromodoris 

 was introduced at the cooler end of the trough. The heated end 

 was toward the light coming from a window facing the sun, and 

 the Chromodoris tended therefore to move into the region of the 

 warmer water. In seven experiments the nudibranchs tested 

 moved promptly toward the light, but ceased moving, elevated 

 the anterior part of the foot from the substratum, and contracted 

 the buccal area when they encountered water of 31° to 32°. 



d. These several hues of evidence point to the existence of 

 well-defined, though not very delicate, thermal sensitivity. The 

 hmiting temperatures (for 'cold' 10°C. and for 'heat' about 

 32°C.) are extreme, so far as the normal experience of the nudi- 

 branchs is concerned. Nevertheless, under natural conditions, 

 temperatures of 31° are met with in shallow water, and even 

 lower temperatures may have a distinctly directive effect. The 

 temperature responses in C. zebra are so vague as to be diflEicult 

 to study carefully; we cannot say that 'heat' and 'cold' receptors 

 are distinct, but it seems probable that 'heat' receptors are dis- 



