SENSORY REACTIONS OF CHROMODORIS ZEBRA 277 



movements of the nudibranchs in nature are suggestive of this 

 possibility (Crozier, '17 b, and section III of the present paper). 

 For reasons subsequently discussed (section III), it seemed ad- 

 visable to test the possibility of a relation between geotropism 

 and the temperature. 



The experiments already referred to were made at different 

 seasons of the year, and the possibility was not lost sight of that 

 the reproductive phase of a given individual might, through in- 

 ternal secretions or otherwise, be instrumental in determining, 

 or in helping to determine, geotropic behavior. The natural 

 movements of C. zebra are of very considerable complexity, and 

 the following statements cannot be applied to the total analysis 

 of these movements. These statements are based upon experi- 

 ence with many hundreds of C. zebra during the last five years. 



During the winter months, at an average laboratory tempera- 

 ture (in the aquaria) of about 17°, C. zebra is notably geotropic, 

 orienting upward and tending to remain at the water surface, 

 especially when about to deposit eggs. This behavior is also 

 notable in the field. After the egg mass is laid, the animal may 

 wander downward again. At 27°, in summer, the same be- 

 havior is manifest, but less pronouncedly. Hence it is unlikely 

 that the decided upward creeping in the first case is the result of 

 oxygen- want. 



The effect of oxygen-want, or of some associated condition, 

 may be tested in two ways: 1) by observing the behavior of ani- 

 mals from which the branchial plumes have been removed or in 

 which these organs are prevented from functioning, and, 2) by 

 observing the behavior of C. zebra on a vertical surface in a jar 

 closed above, containing no air space, but communicating with 

 oxygenated water at its lower end. That the gills are respira- 

 tory organs is suggested not only by their blood circulation, but 

 also by the fact that in sea-water of decreased alkalinity (p^ = 

 7.95-8.00) the gill plumes become widely extended, the base of 

 the gill crown being then inflated and protruded beyond the pro- 

 tecting branchial collar. 



The result of such tests was as follows: Sexually ripe indi- 

 viduals tend to move upward, even though this be away from the 



