As for locular index, it has been believed that each race of the cuttlefish has its own value, though a decline in value of the index with the length of shell was noticed by some reporters. But it seems that both of those conceptions have to be examined.
Therefore 109 specimens of
Sepia esculenta HOYLE caught in Tokyo Bay, in the season from October, 1953 to June, 1954 and in May and June, 1957, were used for morphometric analyses on locular index, and 790 individuals caught during the fishing season in various waters of Japan for reference.
According to the correlation coefficients calculated between the locular index and other three elements, the age in days estimated by fishing date, the number of stripes found on the ventral surface of the shell and the shell length, it was found that the locular index did not decline in value with the length of shell, but with the age or the number of stripes (Fig. 2, 3, 4). Accordingly it might be suggested that the age (fishing date) and the number of stripes adding to the index are very important to serve as standard of comparison between races. And it is obviously desired to make comparison of the index among specimens caught at the same time of the season.
Tracing to the length of shell and last locular at various stages of the growth, left in median longitudinal section of a shell (Fig. 1), a graph of the locular index can be followed through the life for each individual (Fig. 5). By using the graph obtained in such a way, we can easily find a basis of comparison of shell growth between races of the cuttlefish.
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