CEPHALIC EYES — POSITION AND STRUCTURE. 
231 
various degrees of specialization by almost eveiy Gastropod, and still 
])erceptible during develojuueiit in the Pelecypods. They difier from 
the Vertebrate and also from the pallial molluscan eye in the optic 
nerve being distributed posterior to the retinal rods, which are 
directed towards the crystalline lens. In the Gastropoda the cephalic 
eyes are always two in number, usually black or blackish in colour and 
Fig. 451. Fig. 452. Fig. 453. 
Heads of examples of the three chief groups of Gastropods, to show the position of the cephalic eyes. 
Fig. 451, — Vh'ipara vivipay-a (L.), showing the eyes on short pedicels exterior to the tentacles, 
a position especially characterizing the Streptoneurous species (after Moquin-Tandon). 
Fig. 452. — Lim?i(Fa sia^nalis (h.). River Tome, Yorks., showing the eyes at the inner base of 
the tentacles, as illustrative of the Basommatophora. 
Fig. ibZ.— Succhica put?'is Christchurch, Hants., X 3, showing the eyes at the tips of the 
tentacles, as an example of the Stylommatophora. 
placed upon or at the base of the tentacles at the anterior part of the 
body. They are usually more or less spherical, though inclining to 
an elongate shape in the ai^uatic species, but their size bears no 
relation to that of the mollusk, being almost rudimentary in Testacella 
and veiy large in Carijchlum, bnt as usual are })roportionately larger 
in the enibiyo than in the adult. 
The Cephalic eyes of any of onr species are not appreciably inferior 
to the highly organized eye of Helix, which has a very complicated 
structure, the Optic nerve which arises from an anterior lobe of the 
cerebral ganglion being joined with 
the tentacular nerve, from which it 
separates at a variable distance fi’om 
the eye. In Agriolimax agrestis and 
T'estdcelki haUotidea this separation 
takes place quite at the base of the 
tentacle, but in Helix aspersa quite 
within it. The nerve usually contains ganglion cells and expands into 
an optic ganglion before reaching and spreading over the rear of the 
Retina, the part actually sensitive to the impact of light, which occupies 
the rear of the optic chamber, and is a cuticular formation, originating 
from the epithelial cells of the base of the invagination and constituted 
by Retinophores and Retinules,. both of which arise from differentia- 
tions of normal cells and blend together insensibly in character. 
Fig. 454. — Section through partially 
retracted ommatophore of Helix aspersa 
Mull., X 8, with the retractor removed 
to show the position of the eye during 
retraction and the junction within the 
tentacle of the delicate optic with the 
larger olfactory nerve 
