xii PHYLUM MOLLUSCA 771 



portion of the integument covering the exposed face of the eye is 

 termed the false cornea (corn). The eye-ball has a firm wall, or 

 sclerotic, strengthened by plates of cartillage (scl. cart). Externally, 

 i.e., on the side turned towards the surface of the head, this presents a 

 large opening the pupil. The part of the sclerotic which imme- 

 diately bounds the pupil is termed the iris (ir) ; it contains muscu- 

 lar fibres by whose action the size of the pupil can, to a limited 

 extent, be increased or diminished. Just internal to the iris and 

 projecting slightly through the pupil is the lens a dense glassy- 

 looking body of a spherical shape. The lens consists of two plano- 

 convex lenses in close apposition ; it is supported by an annular 



scL.ca.rl 



cil.proc 



com 



scl. cart 



orb.cart 



Fi.-;. tV.S. Sepia, section of eye. cil. proe. ciliary processes ; com. false cornea; ir. iris; lens, 

 lens ; opt. </. optic ganglion ; orb. cart, orbital cartilage ; rds. rods ; ret. retina ; sd. cart, sclerotic 

 cartilage. (From Vogt and Jung, after Hensen.) 



process the ciliary process (cil. proc.) projecting inwards from the 

 sclerotic. Between the two parts of the lens lies a thin layer of 

 cells the cwnea. The lens with the ciliary process divides the 

 cavity of the eye into two portions, a smaller outer the cavity of 

 the aqueous humour, containing water, and a larger inner, contain- 

 ing a gelatinous substance the vitreous humour. Over the wall 

 of this inner chamber extends the retina (ret), the sensitive part of 

 the eye, in which the optic nerve-fibres derived from the optic 

 ganglion terminate. The retina is of somewhat complicated 

 structure, consisting of a number of layers: of these that which 



