Eyes may be absent, as in nearly all the headless Lamellibranchs; but in other Molluscs they are generally present, and may be more or less well developed. An instructive series of stages is exhibited by the Cephalopoda. In _Nautilus_ the eye remains an open pit; in _Ommatostrephes_ two chambers appear, the anterior of which is bounded posteriorly by the lens, and is open to the exterior, so that sea-water enters it; in _Sepia_, finally, the anterior chamber becomes closed in front. We may observe that the eyes of all Cephalopods are at first pit-like, or pass through a stage which is permanent in _Nautilus_, one of the geologically oldest types. Cephalic eyes have recently been noticed in _Mytilus_ and _Avicula_. Eyes of a more complicated structure, which are modified tentacles, are sometimes found on the edges of the mantle in Lamellibranchs (e.g. _Pecten_); these eyes resemble those of Vertebrates, and differ from those of most invertebrate animals in having the fibres of the optic nerve entering the distal and not the proximal ends of the retinal cells. Eyes of a similar construction are to be found on the back of the shellless _Oncidium_, and may be about one hundred in number. Eyes of a remarkable character on the shells of some of the Chitons appear to be modified from tactile organs, and are innervated like the ordinary molluscan eye; they sometimes occur in enormous numbers, more than ten thousand being present on one animal (see wax model, Case 2). [Sidenote: Organ of hearing.] In Cephalopods the ear, like the eye, is known to make its first appearance in the form of an open pit, the mouth of which gradually closes up, leaving only a narrow slit in communication with the exterior. It is probable that in many forms the so-called ear is an organ by means of which the mollusc becomes acquainted with changes in the surface over which it is passing; it is often found deeply imbedded in the substance of the foot, where it forms a closed vesicle. [Sidenote: Sense of smell.] There is no doubt that the carnivorous Gastropoda are gifted with a sense of smell, and throughout the series we observe patches of modified cells of the body-wall (the _osphradium_) which serve either as olfactory organs or as an apparatus for testing the nature of the water of respiration. [Sidenote: The sexes and reproduction.] The sexes are distinct in the most highly organized Mollusca, but are united in the same individual in some of the lower forms, such as Land-Snails, the _Opisthobranchia_ (including the Bubble-Shells, Sea-Slugs, &c.), and in some Bivalves. The reproduction of Mollusca is in all cases effected by means of eggs. In some instances the young are actually hatched within the oviduct of the parent, as in the Freshwater Snails (_Vivipara_); and apparently in most Bivalves the eggs are also retained within the valves until hatched.