W. J. CROZIER 589 



sunlight causes the gill-crown to be very fully expanded, and after a 

 short time (15 minutes) the branchias react very feebly if at all to 

 shading. At 30.5°, no shading response was obtainable in direct 

 sunlight, but there was a slight one in diffuse daylight. 



Thus the branchias of Chromodoris are extended under those con- 

 ditions which also normally determine an increased neuromuscular 

 activity, creeping. The gill protrusion commences, however, in the 

 case of light, before the increased creeping activity is well started; 

 therefore the heightened activity is not the cause of the gill pro- 

 trusion. Moreover, as already stated, the control of the gill-plumes 

 and of the gill-crown as a whole is exercised by a mechanism locally 

 contained, while the photokinetic movements are initiated probably 

 through the eyes. Conversely, low temperatures, darkness, and 

 slightly increased alkalinity, which lead to a decrease in motor ac- 

 tivity, cause the plumes to be retracted to a greater or less degree. 



Only when the protrusion of the plumes is permitted by the tem- 

 perature and light conditions is the reaction to shading possible. 



The response is further controlled by the reaction of the medium, 

 being possible only within the range pH = 7.8 -f and 8.3 ±, the 

 normal range of alkalinity in the habitat of this animal being 7.9 -f 

 to 8.25. 



At the acid end of this range the intrinsic photic irritability of 

 the plumes is much decreased, or inhibited, and at a slightly higher 

 acidity the reflex retraction of the gill crown is inhibited. The 

 "advantageous" features of this state of affairs may be noted: when 

 the CO2 content of the water is greatly increased, the gills become 

 widely spread and no longer interrupt their respiratory function by 

 retracting when shaded; whereas, if the alkalinity of the water is 

 increased (correlated, under natural circumstances, with increased O2 

 concentration), the gill-plumes — "no longer so much needed"— 

 remain concealed within their collared pocket.^ 



Locomotor activity, protrusion of the gills, and production of 

 CO2 continue in water of an alkalinity so low (pH = 7.9) that free 



^ In numerous other instances where animals react to shading the response is 

 characterized by great variability; this type of response should therefore provide 

 a good opportunity for quantitative study. Possibly in other instances also 

 some of the controUing influences here distinguished may be found' operative. 



