278 W. J. CROZIER AND LESLIE B. AREY 



oxygen supply. ' Spent' individuals do not. When the branchial 

 collar was sewed together so that the gill plumes could not be ex- 

 truded, non-geotropic individuals did not tend to creep upward, 

 but remained on the bottom. 



The correlation of egg-deposition with negative orientation 

 was very marked. For example, a group of fifteen nudibranchs 

 had been in the laboratory for four months ; during the last three 

 and a half months of this time they deposited no eggs and re- 

 mained for the most part at the bottom of their aquarium; sud- 

 denly, on the same morning, six pairs were formed, the animals 

 crept up to the w^ater edge, and deposited eggs, after which they 

 wandered aimlessly. 



If the temperature be gradually increased in a vessel in which 

 Chromodoris is creeping upward under diffuse light, the ante- 

 rior part of the foot becomes detached from the substratum 

 when the temperature reaches 29° to 30°C., and if this temperature 

 is maintained the animal creeps or falls to the bottom. This is 

 probably an indirect effect of temperature upon geotropism. It 

 is possible that the reproductive mass, enlarged when ripe, acts 

 as a statolyth; if this is correct, geotropic orientation may re- 

 sult: from, 1) a general increased irritability accompanying 

 sexual ripeness, plus, 2) the mechanical stimulation of the loosely 

 anchored internal organs; on a vertical surface, the animal would 

 then turn away from the side against which these organs pressed. 

 This would result in negative geotropism, as found. Together 

 with positive phototropism (vide infra) and a negative reaction 

 to high temperature, geotropic behavior might, then, be im- 

 portant for the determination of the vertical migrations of the 

 species into shallow water at periods of breeding; it would never- 

 theless be incorrect to say that the animal ''moves into shallow 

 water for the purpose of breeding." 



3. Rheotropism 



In some of the situations where C. zebra abounds, as, for 

 example, in Fairyland Creek, the nudibranchs are well exposed 

 to the possibly directi-^je influence of tidal currents of considerable 



