ntral to the eye, resting on the
capito-pedal cartilage, and supported by the large auditory nerve
which apparently arises from the pedal ganglion but originates in the
cerebral. It has the form of a small sac, 1 to 2 mm. in diameter, and
contains whetstone-shaped crystals, such as are known to form the
otoliths of other Mollusca.

The eye of _Nautilus_ is among the most interesting structures of that
remarkable animal. No other animal which has the same bulk and general
elaboration of organization has so simple an eye as that of
_Nautilus_. When looked at from the surface no metallic lustre, no
transparent coverings, are presented by it. It is simply a slightly
projecting hemispherical box like a kettle-drum, half an inch in
diameter, its surface looking like that of the surrounding integument,
whilst in the middle of the drum-membrane is a minute hole (fig. 3,
u). Sir R. Owen very naturally thought that some membrane had covered
this hole in life, and had been ruptured in the specimen studied by
him. It, however, appears from the researches of V. Hensen that the
hole is a normal aperture leading into the globe of the eye, which is
accordingly filled by sea-water during life. There is no dioptric
apparatus in _Nautilus_, and in place of refracting lens and cornea we
have actually here an arrangement for forming an image on the
principle of "the pin-hole camera." There is no other eye known in the
whole animal kingdom which is so constructed. The wall of the
eye-globe is tough, and the cavity is lined solely by the naked
retina, which is bathed by sea-water on one surface and receives the
fibres of the optic nerve on the other (see fig. 14, A). As in other
Cephalopods (e.g. fig. 33, Ri, Re, p), the retina consists of two
layers of cells, separated by a layer of dark pigment. The most
interesting consideration connected with this eye of _Nautilus_ is
found when the further facts are noted--(1) that the elaborate
lens-bearing eyes of Dibranchiata pass through a stage of development
in which they have the same structure as the eye of
_Nautilus_--namely, are open sacs (fig. 34); and (2) that amongst
other Mollusca examples of cephalic eyes can be found which in the
adult condition are, like the eye of _Nautilus_ and the developing eye
of Dibranchs, simple pits of the integument, the cells of which are
surrounded by pigment and connected with the filamen