W. J. CROZEEE. 587 



reduced to varying degrees. The responses of each individual nudi- 

 branch (forty-five in all) were observed under conditions of gradually 

 diminishing alkalinity. 



The results of these experiments were uniform. At an acidity of 

 pH = 8.0 the reaction when the animal was shaded, both in light 

 from a north window and in bright sunlight, was entirely normal as to 

 amphtude, but the latent interval preceding the response was per- 

 ceptibly lessened; at pH = 8.2 (about the natural alkalinity) the 

 mean latent interval, measured with a stop-watch, was 1.0 second at 

 28°; at pH = 8.0 it was 0.7 second. With still lower alkalinities the 

 following series of events was made out. 



Nature of the response to shading (at 28°C.). 



Normal. Latent period 1.0 sec. 



Amplitude normal. Latent period slightly abbreviated (0.7 sec). 



Latent period lengthened (2.0 to 3.0 sec). The ensuing reaction usually 

 violent and complete. 



A delayed response, like the preceding, but in less than half the animals 

 tested. Sometimes no response. Occasionally a plume or two was 

 seen to react. Branchiae much extended, even in the dark. 



No response whatever under any conditions of illumination. Gill- 

 crown greatly extended (Fig. 2). The branchiae still reactive to 

 tactile stimulation, but merely by individual contraction (curling up). 

 Impossible to cause the retraction of the whole branchial crown. 



Acid (HCl) was added to sea water until the pH was reduced to 

 6.3, and in this medium Chromodoris lived for an hour or more in a 

 closed vessel. The gill-crown, as at pH = 7.6 to 7.7, was much 

 protruded (Fig. 2). 



Alkalinities greater than normal, obtained by allowing algae to 

 remain in the dishes of sea water for an hour or more and also by 

 additions of alkali (NaOH), were found to result in a lessened pro- 

 trusion of the gill-crown. Whereas in diffuse light the gill-plumes 

 were extended to a moderate degree at pH = 8.2, the whole branchial 

 apparatus became retracted and concealed from view at pH = 8.4. 

 At an alkalinity of pH = 8.4 the plumes could not be made to extend 

 by the incidence of direct sunlight. The effect of hyperalkalinity upon 

 the exhibition of a reaction to shading is thus in part a secondary 



