522 ZOOLOGY: W. J. CROZIER 



assertive mating in Chromodoris. If the population is composed of a 

 mixture of pure lines, then one effect of this type of copulation may 

 well be, as in Paramecium (Jennings), the prevention of interlinear 

 crossing. Certain generally accepted ideas regarding the life history 

 of nudibranchs may tend to favor this view. The evidence for the 

 presence of pure lines in the Chromodoris stock is, however, entirely 

 inferential. It would, indeed, be almost impossible to obtain good 

 evidence upon this point, unless, possibly, through a study of the rate 

 of segmentation of the eggs; but the eggs of C. zebra are not well adapted 

 for this work, and it is very doubtful if such evidence could be made 

 conclusive. 



Another, and, I believe, at present better founded, suggestion con- 

 cerning the effect of assertive mating is based upon the fact that the 

 size of the egg-masses, and the number of eggs in each ribbon, as well, 

 probably, as the number of egg masses deposited by each animal during 

 a single season, increase directly with the size of the individual. On 

 grounds of physiological economy remembering that mutual fertili- 

 zation is involved, and remembering also that each animal deposits a 

 number of egg-masses at each spawning season it may be argued that 

 the mating of large individuals is an influence tending to increase the 

 number of larvae beyond that which would result from random pairing. 

 In some other nudibranchs assertive mating, if it occurs, may have a 

 different, or an additional, significance. 



Summary. Mating pairs of the nudibranch Chromodoris zebra are 

 found to exhibit a rather high degree of correlation between the sizes 

 of the two members. This is due to assertive mating, which may con- 

 stitute an important influence tending to increase the numbers of 

 larvae. 



1 Contributions from the Bermuda Biological Station for Research, No. 70. 



2 It was necessary to remove the animals from the water and place them, dorsal surface 

 downward, upon a glass plate. 



