ASSORTIVE MATING IN CHROMODORIS ZEBRA 265 



is situated in almost precisely the same relative position in in- 

 dividuals of all sizes, its distance from the mouth and from the 

 surface of the foot being each proportional to the length of the 

 animal. 



V MATINGS IN MASS EXPERIMENTS 



1. Method and findings. Approximately four hundred speci- 

 mens, freshly collected, were allowed to mate together in lab- 

 oratory aquaria. The experiments were made during the time 

 covered by the collection of the field data. About half of the 

 animals involved had originally been obtained as members of 

 copulating pairs, the other half being collected as single, non- 

 mating individuals. Forty to fifty animals, taken at random 

 but including in every instance a fair representation of the differ- 

 ent sizes of specimens, were put into each of a number of nine- 

 gallon aquaria. The 'crowding together' brought about in this 

 way was intentional. The nudibranchs mate with sufficient fre- 

 quency, at this time of year (April), so that the pairs observed 

 under these conditions did not involve merely the originally 

 non-mating individuals. 



The aquaria were supplied with running seawater, as the 

 nudibranchs will neither copulate nor deposit eggs in still water. 

 The water of the laboratory supply-system was less alkaline than 

 the "outside" seawater, but the chromodorids lived in it for many 

 months. The water inlet was so arranged as to be in about 

 the center of the aquarium, and midway from top to bottom. 

 This precaution was important, as when the incoming water was 

 allowed to form a current starting at the wall of the aquarium a 

 considerable number of the individuals became massed about 

 the inlet. Copulating pairs would under these conditions have 

 been difficult to distinguish with certainty; with the arrange- 

 ment here specified, localized crowding was largely avoided, and 

 it was easy to be sure that a given pair was actually engaged in 

 conjugation. 



There is a very pronounced tendency toward the occurrence 

 of epidemics of pairing. Many conjugating couples occur at 

 about the same time in any one aquarium. This is advan- 



