THE GASTROPODA 20 
retinal or pigmented invagination, still widely open to the exterior, 
whose epithelial cells are covered by a layer of rods, but the 
crystalline lens and vitreous body are altogether absent: this con- 
dition is realised in the Docoglossa. In a more advanced stage of 
specialisation the margins of the invagination become approximated, 
so that the ocular cavity, whose walls are pigmented throughout 
their extent, retains a small external aperture, through which water 
is admitted to bathe the crystalline lens: this condition may be 
seen in certain Rhipidoglossa, viz. Pleurotomaria, the Haliotidae, the 
Trochidae (Fig. 100), the Stomatellidae, and the Delphinulidae. 
Finally, the aperture of the ocular cavity is closed, and the crystal- 
line lens is covered in by two superimposed transparent epithelial 
layers, separated by a transparent layer of connective tissue. These 
two layers are (1) the internal cornea or “ pellucida,” a layer of 
small extent which is simply the anterior continuation of the 
retina, the two together forming the internal wall of the ocular 
sphere ; and (2) the cornea proper, which is external, and continu- 
ous with the tegumentary epithelium. This form of eye is found 
in all the Rhipidoglossa, with the exceptions mentioned above. In 
most other Gastropods the structure of the eye is practically the 
same as in the Rhipidoglossa with a closed cornea, with this differ- 
ence, that the pellucida is more and the pigmented retina propor- 
tionally less extensive. The retinal area becomes more and more 
restricted in proportion as the eye becomes more specialised—e.y. 
the Heteropoda—or ceases to be functional, c.g. Guivillea. There 
is often a blood space above the pellucida, as may be seen in 
Dolium, the Heteropoda, the Elysiomorpha, and the Basommato- 
phora. As regards the function of the eyes, it has been experiment- 
ally demonstrated that aquatic Gastropods are incapable of dis- 
tinguishing the form of objects, while the terrestrial species are 
able to distinguish them at a distance of one or two millimetres. 
The eye becomes rudimentary when it is buried in the integuments ; 
and further, it diminishes in size, though retaining its pigmenta- 
tion, in the following burrowing Gastropods: several Naticidae 
(Natica alderi, Amaura, etc.), various Bullidae (Scaphander, Philine, 
Doridium, Gastropteron, etc.), the Pleurobranchidae, many Nudi- 
branchs, and some Pulmonates, viz. Siphonaria, Auricula miduc, 
and A. judae. The eye may also become rudimentary through loss 
of its retinal pigment, while still retaining its superficial position : 
this is the case in species living in situations beyond the reach of 
the light, whether they be abyssal species (Guivillea) or inhabitants 
of subterranean waters (bithynia pellucida). Finally, regression 
may be carried so far that the eye, as a consequence of functional 
disuse, is wholly wanting in the adult state. This phenomenon 
may be seen in burrowing forms, such as various Naticidae, sundry 
species of Terebra, the Olividae (Olivella, Agaronia, Ancillaria), certain 
