SENSORY REACTIONS OF CHROMODORIS ZEBRA 281 



rhinophoreless individuals assumed a position like that in figure 

 4, and the gills and buccal veil were forcibly distorted to a 

 maximal extent by the current, but no orientation took place. 

 A group of fifteen nudibranchs was then prepared, from nine of 

 which the right 'rhinophore,' and from six the left, was removed. 

 When the current was allowed to impinge on the anterior end 

 of the nudibranch, parallel to the long axis of the animal, in 

 almost every case orientation was prompt, and the bending of the 

 body took place in such a way that the side contracted was the 

 one carrying the intact 'rhinophore.' 



Experiments were also made with more localized currents. 

 A stream of sea-water flowing through a tube of 6 mm. bore at a 

 speed of 4 to 5 cm. per second was allowed to impinge horizontally 



Fig. 4 Showing the posture of the body in a Chromodoris exposed to a tidal 

 current (see text). 



upon the anterior end of C. zebra. To this current normal indi- 

 viduals became promptly oriented, the process occupying three 

 to five minutes (at 17°C.). Animals without ' rhinophores' 

 were conspicuously slow and unsuccessful in orienting away 

 from this current, although the buccal veil and the gills were 

 moved by the current to an equal extent in both cases. The 

 negative orientation to the current did occur in some cases, but 

 only after half an hour or longer. The 'rhinophores' are easily 

 distorted by currents and do not retract when moved in this way. 

 A current of small cross section, affecting only the ' rhinophores' 

 (fig. 5), causes the animal to bend toward the unstimulated side. 

 These results leave no doubt that to currents of adequate 

 velocity the nudibranchs are negatively rheotropic and that the 

 'rhinophores' are the prime receptive organs for this kind of 

 reaction. 



