252 W. J. CROZIER 



3) the distance between the genital papilla a7id the mouth, 

 measured from the external orifice of the genital atrium (center 

 of the papilla) to the center of the mouth, along a horizontal line 

 parallel to the median plane of the body; 



4) the distance of the genital aperture above the surface of the 

 foot. 



Some of these dimensions were studied in independent series of 

 individuals, not included in the series of mating couples, and 

 for other series of individuals, included among the pairs of copu- 

 lating nudibranchs, the weight and the volume were ascertained 

 for each individual. The weights were obtained with a platform 

 belance accurate to 0.1 gm., the nudibranchs being dried and 

 deprived of superficial slime by rolling them gently in a towel 

 before weighing. Volumes were ascertained by the displacement 

 of sea water in a narrow graduated cylinder, the animals being 

 given a preliminary drying as in finding the weight; successive 

 deteniiinations of the volume of the same animal agreed to 

 within 1-2 cc. 



It was not possible to carry out all of these measurements 

 upon each individual, owing to lack of time. My first object 

 being merely to ascertain whether or not assortive mating occurs 

 in Chromodoris, and if so, to measure its intensity, it was con- 

 sidered that the measurements made were sufficient for this 

 purpose. 



A word of explanation is required concerning the choice of 

 points between which measurements were made, particularly 

 with reference to the estimation of total length {A-B, fig. 2), 

 upon which the chief reliance is placed. Under ideal conditions 

 it would probably be preferable to employ the mantle length 

 as defined above. The projecting 'tail' of the foot is contractile, 

 and its length when at rest may vary in a way not closely cor- 

 related with the length of the individual. But with a number 

 of specimens it was not permissible to employ the mantle length, 

 because the animals had suffered a very curious type of natural 

 injury. 



In a preliminary paper concerned with the question of ''warn- 

 ing" coloration in C. zebra (Crozier, '16 b), I stated that the 



