ABSTRACT
The ratio between an arm length (R) and disc diameter (r) has often been used in systematic descriptions of seastars. The values of R and r indicate the size of a seastar, whereas the R/r ratio gives an idea about its shape. In addition, most seastars show allometric growth of the arms with respect to the disc. The ratio of averages of both variables is a correct measurement, although this value tells us nothing about shape change with size. The allometric growth equation: R=a rb is the best way to explain the ratio between R and r. However, two parameters and are necessary to characterize the shape of a given species. Therefore, the R/r ratio of a species varies with the size of individuals. Taking the R/r ratio of each individual as a new adimensional variable is inadequate, since the latter follows a distribution depending on those R and r in a complicated way.
