Growth Zones in the Echinoid Skeleton

Growth zones in echinoid skeletal ossicles are mainly the resultof differences in structural characteristics. In the test plates,opaque zones, which appear light in reflected light, dark intransmitted light, and are X-ray dense, have relatively largertrabecules and smaller intertrabecular channels. Translucentzones, which appear dark in reflected light, light in transmittedlight, and are less X-ray dense, have relatively smaller trabeculesand larger intertrabecular channels. Organic material in theplates, especially when pigmented or charred, enhances the appearanceof the growth zones. Opaque zones result from relatively fastplate growth while translucent zones result from relativelyslow plate growth; food deprivation leads to the formation oftranslucent zones. The growth zones appear to be formed seasonally,at least in some cases, probably in relation to seasonal changesin growth rates, and perhaps in relation to seasonal reproductiveactivity