Page:Proceedings of the Royal Society of London Vol 60.djvu/243

222 The total number of crabs measured was 2300. The determinations 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 indiscriminately 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 0T07 mm.

F IG. 1.— Portunus depurato$ (Plymouth, 1893-95). A

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