(.'EPUALIC EYES — STllUCTriiE. 
'The Retinmilinres aro elongate colourless seusory cells with very cou- 
tractecl free eiuls, and with their bases joined to and continuous with 
tlie nerve pr(.)long'ations : while the Retniules 
Fig. 4.V). 
Fig. 456. 
are pigmented cells with 
Fig. 457. 
Tlnee assumed stages in the evolution from the simple to the more highly organized cephalic 
eyes of Gastropods, showing the retinal rods directed towards the lens and the distribution behind 
them of the optic nerve, and also exhibiting ditTerent degrees of specialization at which further 
development has been arrested in certain species. 
P'ig. 455. — Simple epithelial invagination forming the primitive cephalic eye of Patella^ highly 
magnified (after Hilger). <’/. epithelium ; optic nerve ; y. retina, with superposed retinal 
rods and a mucoid accumulation, which is assumed to represent the origin of the refractive body. 
Fig. 456. — Cephalic eye of Trochus^ highly magnified (after Hilger), showing the arrestment of 
development before the formation of the cornea, etc. t'p. epithelium ; 1. crystalline lens ; op.n. 
optic nerve ; r. retina, with retinal rods. 
Fig. 457. — Cephalic eye of Ifeii.v pofiiatia L., highly magnified (after Simroth), as exemplifying 
the degree of organization attained by the eye of the llritish e.vtra-marine mollusks. c. inner 
cornea ; cu. cutis ; cJ. crystalline lens ; <■/. epithelium, becoming thin and transparent and forming 
the outer cornea ; o.m. outer membrane or sclera ; op.n. optic nerve ; ret. retina ; t.n. tentacular nerve. 
their free extremities very wide, so that they encompass or surround 
the retinopliores, and constitute the i)igmented layer, formerly 
descrihed as the Choroid, hut which does not correspond to the pig- 
mented vascular layer knuwn hy that name in the Vertehrate eye; in 
t [i/cliisfo/i/a eler/xns the ]»igniented retinules are hlled hy black granules, 
Fig. 45S. Fig. 450. 
Fig. 4.)8. — Relinophores or sensory cells and Retinules or pigmented cells from the retina of 
Cyclostonia f Mull.), highly magnified, to show their arrangement (after Garnault). 7'ct. 
retinophore ; rs. rctinule. 
Fig. 450. — Diverse forms of the Retinophores or sensory cells from the retina of Cydostoina 
(Mull.), highly magnified (after Garnault). 
which, when freed from the cells, have active In-owidan movements. 
'I’he Crystalline lens, with the develoi)nient of which the 02)tic vesicle 
Ijeconies a true eye and capable of forming a more or less definite 
image on the retina, is a transparent splueroidal body of concentric 
