The nature and mode of life of the graptolite zooid with reference to secretion of the cortex

Scanning electron microscope studies of well preserved, isolated graptolites have revealed a radically new and unexpected structure to both normal cortex and endocortex, termed here "cortical bandaging". Bandage structure is described and illustrated for several species and the collagenous nature of the· component fibrils confirmed. The evidence presented sheds new light on the nature of the graptolite zooid and how it functioned, in particular on the mode of skeletal secretion. A model involving a more pterebranch-like zooid is preferred to the recent idea of Urbanek (1976) and Kirk (1972) for secretion beneath a perithecal membrane.