280 W. J. CROZIER AND LESLIE B. AREY 



far south (in December) to cause the nudibranchs, not singly, 

 but by the dozen, to migrate southward in this channel. During 

 falling tide, with the current flowing northward, the nudibranch, 

 moved with the current; but once out of the channel they tended 

 to turn around so as to face the sun as soon as they were out of 

 the current. In this way groups of fifty to sixty individuals 

 were caused to collect just beyond the northern end of the little 

 channel. For the study of rheotropic orientation a flat slab of 

 rock was placed horizontally in this channel, and nudibranchs 

 were placed up on it in various positions with reference to the 

 current. In some cases the Chromodoris was allowed to become 

 attached to the rock while surrounded by an inverted glass jar 

 which temporarily protected it from the action of the current. 

 So long as the current was of fair velocity, orientation was always 

 precisely negative, the nudibranch moving with the current. 

 Under these conditions, the whole body is swayed to one side or 

 the other by the force of the current, the gill plumes are moved 

 by it, the ' rhinophores' are bent backward or to one side, and the 

 buccal veil of the mantle is irregularly distorted. The gill plumes 

 and 'rhinophores,' in particular, are forcibly moved by a cur- 

 rent too weak to noticeably affect the body as a whole in a 

 grossly mechanical way, yet leading to precise orientation in 

 the current. It was considered that some or all structures 

 mechanically distorted by the water current might be responsible 

 for the negative rheotropic orientation. 



Experiments were begun with the 'rhinophores.' When 

 exposed to mild water currents of suflicient volume, as in the 

 natural channel already considered, the 'rhinophores' are forced 

 backward (fig. 5). When the current is stronger, the position 

 assumed is as shown in figure 4. The 'rhinophores' are easily 

 removed by seizing with forceps and cutting close to the collar. 

 In one experiment six nudibranchs from which both 'rhino- 

 phores' had been removed the previous day (they crept about 

 in an entirely normal fashion, for removal of the 'rhinophores' 

 has no seriously adverse effects) were found not to be oriented 

 by a current in the natural channel, although a dozen or more 

 individuals with intact 'rhinophores' oriented precisely. The 



