276 W. J. CROZIER AND LESLIE B. AREY 



to maintain an upright position of the body, with the foot ven- 

 tral. The righting behavior is probably due merely to the 

 stereoptropism of the foot, especially at its anterior extremity. 

 A Chromodoris placed on its back will become attached to a 

 glass plate appropriately held in contact with the foot, even 

 though the body remain upside down. This is best tested in C, 

 roseapicta, where the foot does not tend to become folded to- 

 gether. The surface of the foot must be in contact with some- 

 thing. When removed from a substratum the foot folds to- 

 gether longitudinally so that the lateral halves of its surface are 

 in mutual contact. The origin of the twisting movements is 

 probably found in the mechanical excitations of the skin induced 

 by placing the nudibranch on its side or back; the anterior edge 

 of the foot also exhibits writhing movements when the animal is 

 so stimulated, but upon getting into contact with a solid surface 

 it reacts positively, by attachment and slime secretion, and 

 righting is begun. 



Repeated tests have been made to discover good evidence of 

 geotropic orientation in Chromodoris, but without a decisive 

 result being always obtained. Many individuals, in the light 

 or in the dark, creep upward to the water's edge in an aquarium; 

 but they also move downward, horizontally, or in any inter- 

 mediate direction with perfect freedom. When situated on a 

 glass plate which was tilted in various" directions, they continued 

 creeping 'as they were,' and could not be made to alter at the 

 experimenter's will the direction of their creeping. These ex- 

 periments were made at temperatures of 17° to 27°C. There 

 seemed a somewhat more pronounced tendency to upward move- 

 ment at 17° than at 25° to 27°, but the difference was not clear- 

 cut and is perhaps fictitious. ^ 



If C. zebra possesses statolyths (otoconia) similar to those 

 known in other nudibranchs, and perhaps of general occurrence 

 in the group, they are not conspicuously involved in determin- 

 ing the direction of the animal's movements in the laboratory, 

 nor the posture of the body in nature. It is of course conceiv- 

 able that a vaguer type of geotropism is really functional, which 

 might be difficult to detect in laboratory experiments. The 



