The fine structure of the sense organs of the cephalopod mollusc Nautilus

Summary The structure of the rhinophore, digital tentacles, post-ocular tentacles and the eye of Nautilus macromphalus are described. The rhinophore is composed of mucous cells, ciliated cells, and flask-shaped ciliated cells. The latter are probably olfactory receptors. The digital tentacles are composed of mucous cells and pigmented cells. Motor-end-plates found in the muscle layer below the epithelium of the digital tentacles are similar to those described in other cephalopods. The post-ocular tentacle contains receptor cells that bear macrocilia. These may be mechanoreceptors. The retina is composed of retinula cells and supporting cells. A complex rhabdom is formed at the distal ends of the retinula cells. The supporting cells send processes up between these rhabdoms. Both types of cells contain pigment granules but the retinula cell has a complex membranous structure in its perikaryon. No synapses were found at the bases of the retinula cells. At the side of the retina are mucous cells that are presumed to produce the jelly-like substance that fills the inside of the eye in life. The likely function of the eye is discussed and it is suggested that it is capable of simple discriminations. It is suggested that the sense organs are probably comparatively unchanged from those of fossil nautiloids. Acknowledgements. This paper is dedicated to the late Dr. Yves Merlet who collected the nautiluses used in this study.We would like to thank Prof. J. Z. Young for all his support and encouragement. The Royal Society, The Percy Sladen Memorial Fund, and University College, London, provided the financial support that enabled one of us (V.C.B.) to collect nautiluses. The Science Research Council, U.K., provided the electron microscope used in the major part of the study and a grant to one of us (V.C.B.). We would also like to thank Prof. J. B. Gilpin-Brown who provided Fig. 1, Dr. R. Catala, for aquarium facilities, Mr. M. P. Legand and the Institut Français d'Oceanie, Noumea, New Caledonia, for laboratory facilities, Dr. J.-M. Bassot and Dr. Anna Bidder for advice on catching and preserving nautiluses, Mrs. Judy Parkes and Mr. M. Barker for photographic assistance, and Miss J. Date for secretarial assistance.