It is interesting to note that whereas in the cuttle-fishes and most molluscs, the rods of the retina are turned towards the light, in _Pecten_, _Onchidium_ (a kind of slug), and some others, they are, as in vertebrates, turned from the light. In _Pecten_ the nerve to supply the retina bends round its edge at one side. But in _Onchidium_ it pierces the retina as in vertebrates. In worms, eyes are sometimes present, sometimes absent. In star-fishes and their allies they often occur. In medusæ (jelly-fish) they are sometimes found on the margin of the umbrella. Even in lowly organisms, like the infusoria, eye-spots not unfrequently occur. We must remember, however, that, in these lower forms of life, the organs spoken of as eyes or eye-spots merely enable the possessor to distinguish light from darkness. Even when eyes or eye-spots are not developed, the organism seems to be in some cases sensitive to light--using the word "sensitive," once more, in its merely physical acceptation. The earthworm, for example, though it has no eyes, is distinctly sensitive to light; and the same has been shown to be the case with other eyeless organisms. Graber holds that his experiments demonstrate that the eyeless earthworm can distinguish between different colours--in other words, is differentially sensitive to light-waves of different vibration-period--preferring red to blue or green, and green to blue. And the same observer has shown that animals provided with eyes--the newt, for example--can distinguish between light and darkness by the general surface of the skin. M. Dubois, by a number of experiments on the blind _Proteus_ of the grottoes of Carniola, has shown that the sensitiveness of its skin to light is about half that of its rudimentary eyes; and, further, that this sensibility varies with the colour of the light employed, being greatest for yellow light.[FL] We have not been able to do more than make a rapid survey of the sense of sight as it seems to be developed in the invertebrates and lower animals. The visual organs differ, not only in structure, but in principle. We may, I think, distinguish four types. 1. Organs for the mere appreciation of light or darkness (shadow), exemplified by pigment-spots, with or without concentrating apparatus. 2. Organs for the appreciation of the direction of light or shadow, with or without a lens. The simple retinal eyes of gasteropods, and perhaps in some cases the ocelli of insects, probably belong to this class. 3. True eyes, or organs in which a retinal image is formed, through the instrumentality of a lens, as in vertebrates and cephalopods. 4. The facetted eyes of insects, in which a stippled image is formed, on the principle of mosaic vision.