272 W. J. CROZIER 



There is evidently some influence other than chance contact 

 which is at work to bring mating individuals together. Shore- 

 ward movements out of deep water at certain periods (Crozier, 

 '17 b), which nevertheless do not appear to be carried out ''for" 

 purposes of reproduction, play a part in this process. So do the 

 reactions leading to the concentration of individuals in the man- 

 grove creeks and upon open sandy bottoms. In one particular 

 place it has been clear that water currents have been important 

 in bringing a number of individuals to come near to one another. 

 This spot is along one border of a shallow strip across which there 

 is a water connection between two arms of Fairyland Creek. 

 A row of flat stones is situated along the north edge of this 

 strip, and with the change in the tide a strong current flows over 

 the narrow neck. A number of chromodorids were usually 

 found in pockets hollowed out by the current underneath the 

 stones referred to. This is probably the result of their being 

 ''entrapped" there at periods when the current is not strong; 

 they move into the "pockets," or are forcibly carried there, in a 

 somewhat passive fashion, as the animals are not rheotropic. 

 This is the only place where I have found C. zebra under stones; 

 it paired in this situation, and deposited eggs. Unlike some 

 other species of this genus, C. zebra moves toward the light. 

 This positive phototropism is in itself a factor tending to bring 

 into company a large number of individuals. 



It is a noteworthy fact that these nudibranchs are very sel- 

 dom found to occur singly. If only a very few specimens are dis- 

 covered in the course of a day's collecting, it is nevertheless usu- 

 ally found that the animals tend to occur in groups, the indi- 

 viduals being relatively near together. When the nudibranchs 

 are actively engaged in breeding, groups of specimens, such as 

 are illustrated in the following records, are not infrequent: — 



May 2nd 1917. Groups of C. zebra observed in Fairyland Creek. 

 All the individuals in each group were closely packed together, each 

 group being well separated from all the others. The individuals mat- 

 ing together in each group (when copulating couples could be distin- 

 guished with certainty) are bracketed together. The measurements 

 are those of 'total length,' in cms. 



