406 PHYLUM MOLLUSCA. 
The following chief nerves arise from the central system :— 
(1) The very thick optic nerves are given off from the commissures 
between cerebrals and pleuro-viscerals, and lead to a large 
optic ganglion at the base of each eye. is 
(2) Ten nerves to the ‘‘arms” are given off by the pedal ganglion, 
and this is one of the reasons which have led most morph- 
ologists to regard these arms as portions of the ‘‘ foot.” 
(3) Two large nerves from the more ventral portion of the pleuro- 
visceral ganglia form a visceral loop, and give off many 
branches to the gills and other organs. From the pleural 
portion arise two mantle nerves, each of which ends in a 
large stellate ganglion. 
Sense organs.—The eyes are large and efficient. They present a 
striking resemblance to those of Vertebrates, and, as they are not ‘‘ brain 
eyes,” they illustrate how superficially similar structures may be 
developed in different ways and in divergent groups. In cuttlefishes 
the eyes lie on the sides of the head, protected in part by the cartilage 
surrounding the brain, and in part by cartilages on their own walls. 
The eye is a sensitive cup arising in great part from the skin. Its 
internal lining is a complex retina, on the posterdor surface of which the 
nerves from the optic ganglion are distributed. It seems likely that the 
Cephalopod retina corresponds only to the rods and cones (the sensory 
part) of the Vertebrate retina. In the cavity of the cup there is a clear 
vitreous humour. : 
The mouth of the cup is closed by a lens, supported by a ‘ciliary 
body.” The lens seems to be formed in two parts—an outer and an 
inner plano-convex lens. The pupil in front of it is fringed by a con- 
tractile iris. 
The outer wall of the optic cup is ensheathed by a strong supporting 
layer—the sclerotic, which is in part strengthened by cartilage, covered 
by a silvery membrane, and continued into the iris. 
In front of the eye there is a transparent cornea, and the skin also 
forms protecting lids, 
Round about the optic ganglion there is a strange ‘‘ white body,” 
which seems to be a fatty cushion on which the eye rests, 
The two ear-sacs, containing a spherical otolith and a fluid, sometimes 
with calcareous particles, are enclosed in part of the head cartilage, 
close to the pedal ganglia. The nerves seem to come from the pedals, 
but it is said that their fibres can be traced up to the cerebrals. 
A ciliated ‘‘ olfactory sac” lies behind each eye, and is innervated 
from a special ganglion near the optic. There are no osphradia of the 
usual type. 
Finally, there are tactile or otherwise sensitive cells on various parts 
of the body, especially-about the arms, 
Apart from sight altogether, an octopus can find a dead fish at a 
distance of over a yard in a few minutes, and even slight movements in 
the water are detected. 
In many Decapods there are luminous organs, usually on the ventral 
surface in diverse positions, and often buried. They may serve as 
recognition-marks or as search-lights. They may be glandular or 
