254 W. J. CROZIER 



couples obtained, the Hotal length' measurement was mainly 

 used. 



The length of a Chromodoris as measured in the way described 

 is somewhat greater than that obtained from the quietly creeping 

 specimen. The body is of but little greater density than sea 

 water (1.062-1.072 gms./ccm., as compared with 1.0265), but 

 is so soft that when adjusted upon the measuring plate its weight 

 is sufficient to noticeably flatten the animal. The distortion in- 

 duced in this way is probably greater, proportionally, in the case 

 of large individuals, weighing up to 75 gms., than in the smaller 

 specimens, which weighed 10-15 gms. The flattening of the 

 body through the pressure of its own mass, under the conditions 

 imposed during the measurement, helps to obscure the smooth- 

 ness of the expected theoretical relation between length and 

 volume (fig. 4). It should also be remarked that the nudi- 

 branchs may tend to elongate somewhat, by muscular contrac- 

 tions, as a preliminary to righting movements. The volume and 

 weight of a Chromodoris may also vary independently of the 

 length, owing to the influence of such factors as: 1) the number 

 of egg masses which have been deposited, and 2) the amount of 

 water contained in the intramuscular spaces of the foot and 

 body-wall. Nevertheless, the qualitative agreement between 

 the indices of correlation in mating calculated from the several 

 kinds of measurements taken shows that the estimations of 

 ' total length' as here carried out are adequate for the determina- 

 tion of assortive mating with respect to size. The charac- 

 teristic reaction of the foot when removed from contact with a 

 flat substratum, namely the rolling up into close approximation 

 of its lateral borders in the median plane (fig. 2), helped to 

 insure a uniform attitude among the various individuals during 

 measurement. 



IV ASSORTIVE MATING IN NATURE 



1. Total length. No significant difference was observed be- 

 tween the size of the nudibranchs obtained as members of con- 

 jugating couples and those taken singly, either in the collections 

 of any one day (illustrated by the data in figures 5 and 6), or 



