ASSORTIVE MATING IN CHROMODORIS ZEBRA 273 



To what extent 'attractive' secretions may be involved in 

 producing these groupings, I am as yet unable to state. The 

 matter will be studied further, as it is of interest, among other 

 things, to determine why the several species of Chromodoris 

 cannot be made to conjugate with one another. C. zebra does 

 possess a characteristic and very penetrating odor. 



The details of copulations were examined in the laboratory. 

 When two individuals come near together it is frequently to be 

 observed that one of them is stationary, the other moving toward 

 it. This occurs in still (non-circulating) water. When they have 

 come within about 10 cms. of each other, if not before, the 

 genital papillae are frequently protruded. In many instances the 

 protrusion is evidenced by one of the animals earlier, or more 

 strikingly, than by the other; in a number of cases it was noted 

 that the former individual was relatively stationary, the latter 

 moving toward it. Often the animals come together either with 

 their left sides in apposition, or 'head on;' in either event they 

 slowly move about until finally, headed in opposite directions, 

 the surfaces of the right sides are in contact. This operation 

 may require as much as an hour, or longer. The genital papillae 

 become gradually more prominent, and exhibit movements of 

 extension and partial retraction which are more or less rhythmic. 

 Not uncommonly the protrusible pharynx is everted, and the 

 lips are passed over the surface of the other animal. This be- 

 havior has also been observed in the field. 



When the two individuals are of nearly the same size, they be- 

 come fitted to one another after the manner sketched in figure 16. 



THE JOCRNAL OF E.XPERIMEXTAL ZOOI.OOY, VOL. 27, NO. 2 



