WIDTH IN MILLIMETERS FIG. 2. Showing growth trends In Callinectes sapidus Rathbun, based upon growth curve of Fig. 1. there are believed to be 19 or 20 in males and 18, or pos-sibly 19, in female blue crabs (7). A mathematical equation was fitted to the molting curve for males and females so that from any initial width one can calculate directly the final width after molting. Hence, knowing the width of the first postlarvalinstar, it was possible to calculate the widths of the remaining instars. To complete the story, it was necessary to establish the relationship between the width dimension, selected for a basic index of size, and other linear and also weight measurements. The allometry equation Y= aXb, was found to constitute a satisfactory expression of the different linear and also weight relationships. Conse-quently, from a width measurement, any of the corre-sponding linear and weight dimensions can be readily ascertained. Since the width of each instar has been determined, it is now possible to calculate the remaining linear dimensions and the weights of each instar (Fig. 2). The information provided by this procedure makes it possible to analyze certain characteristics of the growth of body parts in relation to one another and to the whole body. Also, a basis is provided for interpreting the effect of environmental conditions and mechanical injury