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).
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.
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.
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.
The ova of many molluscs are deposited in masses enclosed in capsules. Some of them are very wonderful and complicated structures. Those of the Cuttles and their allies are clustered like grapes, each capsule containing but a single embryo; but in the Calamaries or Squids they form a radiating mass of elongated sacks, each containing from thirty to two hundred eggs, and it has been estimated that one of the spawn-clusters of the Common Squid (Loligo vulgaris) contains as many as 40,000 ova. Everybody knows the spawn-cases of the Common Whelk, found so abundantly on the sea-beach, consisting of a large number of yellowish capsules, heaped one upon another and forming an irregularly rounded mass. As many as five or six hundred capsules may be piled together in a single heap, each capsule containing several hundred eggs, of which perhaps only thirty or forty are hatched.