AbstractThe structure and function of the keratinised teeth ofMyxine glutinosahave been studied. Following a detailed histological examination, histochemical, ultrastructural and experimental techniques have been used to study some aspects in more detail. A modification of the standard glutaraldehyde fixative for electron microscopy has been developed forMyxinetissues.The development of the horn cap (HC) and the pokal cell cone (PCC) have been traced. The HC is seen to increase in thickness with increasing length of the animal but there are irregularities in the development of the PCC which suggest that this cone may act as a replacement tooth for the HC. Experiments, in which the HCs were removed and the changes in the PCC observed, were performed to test this hypothesis. As a result of these and other observations it is suggested that it is possible for tooth replacement to occur.The structure and formation of the keratin in the HC and PCC cells has been followed using electron microscopy and histochemistry. The accumulation of keratin fibres in the early stages of PCC development has been traced, and keratinisation, which is probably associated with an increase in height of the cusps has been followed across a keratogenous zone at the bases of well developed HCs and PCCs.It has been established that the HC pigment granules are lipofuscin and that they can originate from lipid droplets.A detailed investigation has been made of the pokal cells. It is suggested that the beaded fibres, observed in electron micrographs of the apical band of these cells, are synthesised from lipid droplets in basal portions of the pokal cells, in a synthetic cycle, during which the nucleus descends through the length of the cell.