Abstract
Analyses of the form in various organisms have been presented in recent years by Huxley and his students. Certain related findings incidental to a study of the life-history and growth of the Pacific Edible Crab (Cancer magister Dana) are here given. The data include measurements of various linear dimensions (carapace length and width, length of chela, length of the first walking leg, length and width of the sixth and seventh abdominal segments in both the male and female) of 1804 crabs. The analysis has shown the relation of various sexual differences to the onset of sexual maturity and has suggested certain correlations between body proportions and the size attained by the species.
Treated by Huxley's method of plotting the logarithm of the part on the logarithm of the whole 1 (here usually the carapace width) in most cases one or more straight lines are obtained. The slopes of these lines are the values of k, the differential growth ratio. The nature of this constant may be seen from the following formula. If the plot may be fitted by a straight line the equation of this will be
log p = log a + k log w
where p represents the part considered, w the whole, here the carapace width, and a and k are constants. Transforming, this becomes
p = awk
The values obtained in Cancer magister range from 0.93 to 1.61. These do not differ greatly from unity, representing isogony or growth with constant form; they should be contrasted with the values found by Shaw2 and Huxley in Inachus, Carcinus and Pachygrapsus which range between 0.32 and 3.15.
The differences between the sexes in the edible crab are of 2 kinds. Differences in the primary sexual organs and in certain of the secondary sexual characters are present in the earliest post-larval stages. With the onset of sexual maturity other secondary characters change in one or the other sex producing differences, or differences already present may become accentuated.
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