STRUCTURE OF SEPIA. 335 



(2) The nerves to the ' ' arms " are given off by the pedal ganglion, 



and this is one of the reasons which have led most morpho- 

 logists to regard these arms as portions of the "foot." 



(3) Two large visceral nerves from the pleuro-visceral ganglia 



form a visceral loop, and give off many branches to the gills 

 and other organs. From the same source arise two mantle 

 nerves, each of which ends in a large stellate ganglion. 



Sense-Organs.—'Y\i& eyes are large and efficient. They 

 present a marked but by no means perfect resemblance to 

 Vertebrate eyes, and thus illustrate how the same sort of 

 structure may be developed (surely not by the natural selec- 

 tion of indefinite fortuitous variations), 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. The internal surface of this cup is a complex sensitive 

 retina innervated from the optic ganglion. In the cavity of 

 the cup there is a clear vitreous humour. 



The mouth of the cup is closed by a light-focussing lens, 

 supported bya "ciliary process." The lens seems to be formed 

 strangely enough in two parts — an outer and an inner plano- 

 convex lens. The pupil or hole in front of the lens is fringed 

 by a contractile 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. 



There is no true cornea, at least not like that of the 

 Vertebrate eye, but the skin forms a complete lid over the 

 eye, and in this lid there is a little hole by which water enters 

 and bathes the outer part of the lens, as the aqueous 

 humour does in our eyes. 



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 by the pedal ganglia, but 

 are apparently innervated from the cerebrals. 



