﻿194 PROCEEDINGS OF THE NATIONAL MUSEUM voi-. lOo 



The ratio of width over length, i. e., constant 6, in the above growth 

 equation, is not the same in the two sexes. From the following list 

 of requisite statistics : 



Male Female 



Number of individuals 107 71 



Mean width/length ratio 63.45% 64.92% 



Standard deviation 0.498 1.979 



Standard error of mean 0. 048 0. 234 



Coefficient of variation 0. 786 3. 03 



the difference in the mean values of this ratio between males and 

 females is found to be 1.47 percent and the standard error of this 

 difference 0.24. As the difference is more than six times its standard 

 error, it may be concluded that the females as a whole have broader 

 carapaces than the males, though in neither sex does the ratio between 

 width and length alter appreciably with growth. 



Figure 28. — Length-width curves of Jrgulus japontcus from Erh Hai (o = locus for more 



than one point. 



In comparing the carapace, abdomen, and anal sinus only individ- 

 uals larger than 3.5 mm. in length are used so as to exclude the im- 

 mature forms. In table 6 the carapace and abdomen of the two sexes 

 are compared with reference to absolute length and the ratio of width 

 and length of these organs. As the two sides of the carapace are gen- 

 erally not of the same size in fixed material, by length of carapace is 

 meant the average of the left and right lengths measured. 



It will be seen from table 6 that these two structures in the female 

 are slightly more variable than those in the male, as shown by their 



