266 
In both orders of the Class the Head is well developed, 
and provided with highly organised sensory organs ; fore- 
most among these are the Eyes ; * they are usually large 
and complex. The Dihranchiates have eyes which are lodged 
in orbital cavities ; each eye possesses an optic nerve, a sclerotic, 
choroid, retina, vitreous humour, aqueous humour, and lens. 
It must, however, be remembered, as Professor Huxley points 
out, that these apparent resemblances between the cephalo- 
podous and the vertebrate eye are merely superficial, and dis- 
appear on detailed comparison." The Tetr abranchiate eyes are 
of much simpler construction ; they are not placed in orbital 
cavities, but are attached by stalks or pedicels to each side of the 
head, and consist of simple cup-like organs lined with a 
retina. 
The Olfactory OrganSy which are represented by little 
depressions, and by minute soft prominences of the skin, which 
are situated near the eyes, either above them or below them, 
have been investigated and described by Kolliker. 
The Auditory Organs in the Dihranchiata are lodged in the 
cartilaginous case which protects the great nervous centres, 
and consist of cavities each of which is furnished with a 
single otolith. In the order to which the !N'autilus belongs 
the auditory capsules were first discovered by Dr. Macdonald. 
" The inner wall of each ear-sac in the JN'autilus is somewhat 
flattened, lying in contact with the nervous matter, but its 
more convex external surface rests in a little depression on 
the upper and internal border of the cephalic cartilage. It 
is enveloped in a kind of fibrous tissue, and filled with a 
cretaceous pulp, consisting of minute elliptical otokonia, 
which, when under a high power, present a bright and 
* For the anatomy of the eye in Cephalapoda see " Beitrag zur nahern Kcnnt, 
niss des auges der Cephalopoden von, Dr. Krohn, 1833." " Hensen, ueber das 
auge einiger Cephalopoden, in Siebold und IvoUiker Zeitschi-ift fUr wissenscbs 
Zool," 1865 ; and " Structure of the optic lobes of the Cuttle Fish." J. L. Clarke, 
London, 1866. 
