ASSORTIVE MATING IN CHROMODORIS ZEBRA 249 



desirable to emphasize the probably mechanical natm-e of the 

 origin and maintenance of this 'adaptive' condition, which can 

 be shown to depend upon the way in which the body of Chromo- 

 doris is constructed. 



II MATING PAIRS UNDER NATURAL CONDITIONS 



The copulating couples used in this study were gathered dur- 

 ing the month April 5 to May 5, 1917. All the specimens were 

 taken in Fairyland Creek, a broad, shallow, tidal stream bor- 

 dered by mangroves, near the laboratory of the Bermuda Bio- 

 logical Station. Collecting was purposely restricted to this one 

 locality. The nudibranchs are very plentiful there, at certain 

 times. Chromodoris copulates at all hours of the day, but it 

 seems that a somewhat larger number of pairs are obtained here 

 during the three or four hours after sunrise, and again in the 

 late afternoon, than during the middle of the day. The state 

 of the tide has, in some places, a secondary influence upon the 

 occurrence of mating, the creek being so shallow that in certain 

 spots the bottom is exposed to the air at low water; the free 

 movement of the nudibranchs is thus restricted, and they are 

 forced to lie hidden under the thick masses of Zostera, Halimeda, 

 Laurentia, and associated plants which cover the greater portion 

 of the muddy bed of the creek. When they are free to move 

 about, theu' positive phototropism^ assists in causing them to 

 appear freely upon brightly illuminated areas, where their con- 

 spicuous coloration renders them easily visible. 



As each pair was secured it was deposited in a separate aqua- 

 rium jar, the different pairs being carefully kept distinct until the 

 dimensions of the animals had been ascertained, after the return 

 to the laboratory. Non-mating animals, found creeping alone 

 over the bottom, were frequently collected at the same time; 

 these individuals furnished a basis upon which to compare the 

 size of conjugants with that of non-conjugants, and also sup- 



2 The phototropism of C. zebra is not in any sense a "laboratory product," 

 but on the contrary a very real element in its behavior under conditions of free- 

 dom in nature. Some account of the natural movements and behavior of C. zebra 

 will be given in another place. 



