222 



Mr. E. Warren. 



The total number of crabs measured was 2300. The determina- 

 tions were made with compasses and a 3-decimetre ivory scale divided 

 into half millimetres. The measurements were recorded to the tenth 

 of a millimetre. As a test of accuracy, fifty crabs were indiscrimi- 

 nately taken out of a large number which had previously been 

 measured; these were remeasured, and the results compared with 

 those before obtained. It was found that the mean difference between 

 any two measures of the same dimension was 0'107 mm. 



FIG. 1. Portunus depurator <? (Plymouth, 1893-95). 

 A 



Continuous outline represents a crab with carapace length AB = 28'5 mm. Dotted 

 outline represents a crab with carapace length AB = 46*8 mm. 



Since the crabs were growing, and so varied much in size, it was 

 necessary to reduce the measures to percentages of some standard 

 dimension. The carapace length (AB) was selected, and all the 

 measurements are expressed as thousandths of this dimension. 



The first point to ascertain is whether the mean of the different 

 " organs " varied as the crab increased in size. To determine 

 this, the crabs were sorted into groups, according to the length 

 of their standard dimension, then the arithmetic mean was found 

 for the remaining six dimensions. In each group the change 

 in length of carapace was comparatively small, and had to be 

 neglected. The following table gives the results which were thus 

 obtained : 



