In the coronal plates of some echinoid species, orientation of calcite c-axes is a function of growth. In Hemicentrotus pulcherrimus, the first formed plates near the peristome have c-axes that are nearly perpendicular to the plate or test surface; plates of intermediate age (near the ambitus) have axes inclined at an angle of approximately 45 degrees ; and the axes of the youngest plates (near the apical system) are again nearly perpendicular. In Lytechinus variegatus a different pattern obtains but a gradual change in orientation with growth is again observed. Other echinoid species studied do not exhibit ontogenetic variation in calcite crystallography. It is suggested that the adaptive value of a given preferred orientation of c-axes is related to the organism's light sensitivity. Light will pass through a plate having a tangential c-axis more readily than through one having a more nearly perpendicular c-axis. Littoral echinoids with tangential c-axes commonly cover their tests with debris as a shield against light. Adaptation to light conditions in shallow water through changes in relative developmental rates (in species like H. pulcherrimus and L. variegatus) may be the explanation for the evolution of one preferred orientation from another in the geologic past.