460 



PHYLUM MOLLUSCA 



with difficulty. All are well protected by the investing 

 cartilages. 



The cerebral ganglia are three-lobed, and are connected anteriorly by 

 two commissures with a " supra-pharyngeal " ganglion, which gives off 

 nerves to the mouth and lips, and is connected also with an " infra- 

 pharyngeal " ganglion. The cerebral gangha are also connected by 

 short double commissures with the pedals and pleuro-viscerals on the 

 ventral side of the gullet. The pedal gangha at each side are in part 

 divided into two— one half forming the brachial ganglion which sends 

 nerves to the arrjis, the other the infundibular which supplies the 

 funnel. 



The following chief nerves arise from the central system : — ■ 



(i) 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. 



(2) Ten nerves to the " arms " are given off by the pedal ganghon, 



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 gangha 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 posterior surface of which the 

 nerves from the optic ganglion are distributed. It seems likely that the 

 Cephalopod retina corresponas 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, 



