PALLIAL AND DORSAL EYES. 
233 
structure and somewhat gelatinous consistency, secreted by the 
retinal epithelium and surrounded partially or entirely by a more 
fluid cuticular substance, the Vitreous body, these refractive parts of 
the eye being probably primitively represented by a mere accumulation 
of mucus within the integumental invagination. The transparent 
outer Cornea is formed by an extension of the outer integument over 
the optic vesicle and varies in convexity 
and jn’ominence, while the inner Cornea 
or pellucida is immediately beneath and 
formed by the colourless internal wall 
of the ocular bulb. The Sclerotic is the 
bard structureless membrane often 
difficult to separate fr’om the cornea 
which surrounds and gives to the eye its 
distinctive form. 
By disuse, owing to the adoption of 
special or peculiar habits of life, the eyes 
may, as in Carilioides, become diminished in volume and buried in 
the integument, or may retain their superficial position and degenera- 
tion arise from the loss of the pigmentation. 
The Pallial and Dorsal Eyes or Ocelli are secondarily-acquired 
visual organs, difl’ering totally in many respects from the more primitive 
cephalic eyes, and conforming in important features to the structure 
of the eyes in Vertebrates. They are especially developed in the 
Pelecypoda on the more exposed parts of the mantle margins, owing to 
the primitive cephalic eyes, temporarily present during development 
and during the free-swimming larval stage, becoming atrophied and 
lost, as when overgrown and enclosed by the mantle and shell they 
are useless. In Ouchidium, a genus of marine pulmonates, dorsal eyes 
are developed, in addition to the cephalic eyes, in the forms in- 
habiting certain districts which are coincident with the range of the 
Feriophthalinus, a leaping shore fish, which preys upon them. 
In the Pelecypods the pallial eyes have originated upon the nerves 
arising from the innumerable small ganglia of the marginal pallial 
plexus, and may be little more than pigmented spots, acutely sensible 
to light, scattered upon the more exposed parts of the mantle, and, 
as in Area, may, by their combination in groups, form composite eyes 
resembling in structure certain simple Arthropodan eyes, each con- 
stituent eye or ommatidium being formed of a conical visual cell, 
Fig. 4G0. — Optic bulb of Helix 
poviaiia L., after maceration, x 25 
(after Simroth), showing the arrange- 
ment of the pigmented cells. 
