250 W. J. CROZIER 



plied a part of the material for mating experiments. Each 

 time that a collection was made it was attempted to secure all 

 the mating pairs present within the area considered, no selection 

 being exercised other than that involved in deciding whether or 

 not a given couple was actually engaged in copulation; as a rule 

 no difficulty was experienced upon this point. The correlation 

 tables (vide infra) include data on all the pairs found during the 

 interval April 5-May 5, with the exception of three pairs neces- 

 sarily rejected because one or both members were, owing to in- 

 jury (as subsequently explained), unfit for measurement. 



The animals were measured on the same day as that on which 

 they were collected, and usually within a few hours after their 

 removal from the sea. This was thought necessary since certain 

 dimensions might have been altered if the nudibranchs had 

 been allowed time in which to deposit their large egg masses. 

 When, in the process of collection, a copulating couple is dis- 

 turbed, one or both of the individuals, without subsequent con- 

 jugation, may within the next twenty-four hours deposit a large 

 or a small (abnormal) egg mass. 



Ill MEASUREMENTS 



The body of a Chromodoris zebra is soft, contractile, within 

 certain limits easily changed in shape, and without any hard 

 supporting skeleton. The accurate estimation of its dimensions 

 therefore presents a certain amount of difficulty. It was not 

 practicable to get consistent results by the measurement of 

 length while the animal was creeping freely over a smooth hori- 

 zontal surface. Bj^ removing the specimen from the water, how- 

 ever, and placing it, dorsal surface downward, upon a hori- 

 zontal plate of smooth glass freshly wetted with seawater,' it 

 was possible to get length measurements which could be dupli- 

 cated in successive trials to within 0.3 cm. Since the mean 



2 On a dry surface, or upon a glass plate wetted with seawater which has 

 been allowed to evaporate somewhat, the nudibranchs were stimulated to restless 

 contortions. When treated in the manner above outlined they become extended 

 to their maximal length and remain so, quietly, for several minutes, during 

 which the length of the body can be measured with fair precision. 



