Among the deep-sea gasteropods we find the same irregularity in the possession of eyes that we have just described among fishes. Thus a species of _Pleurotoma_, dredged by the ‘Porcupine,’ in 2,090 fathoms, has a pair of well-developed eyes on short footstalks, but _Pleurotoma nivalis_, obtained by the ‘Talisman,’ is blind. Again a species of _Fusus_, obtained by the ‘Porcupine,’ in 1,207 fathoms, is provided with well-developed eyes, but _Fusus abyssorum_, obtained by the ‘Talisman,’ is blind. Among the Lamellibranchs there are very few genera that possess well-marked eyes. The genus _Pecten_ is one of those that in shallow waters possess numerous highly complicated visual organs situated on the edge of the mantle. In the deep-sea species, _Pecten fragilis_, these eyes are wanting, but we have not sufficient evidence at present to enable us to assert that all the deep-sea species of this genus are blind. Among the Crustacea there is a very general tendency to lose the eyes at a depth of a few hundred fathoms of water. In _Ethusa granulata_, for example, the eyes disappear at 500 fathoms and the eye-stalks become firmly fixed, greater in length, and take the place of the rostrum which disappears. In some forms—such as _Thaumastocheles zaleuca_ and _Willemoesia_—the eye-stalks themselves have completely disappeared. In the deep-sea Isopoda some forms lose their eyes entirely, but _Bathynomus giganteus_ possesses a pair of enormous eyes, each provided with 4,000 facets. To illustrate the distribution of eyes in this group, we may take as an example the genus _Serolis_. All the species of this genus are provided with eyes except _Serolis antarctica_—a species that extends from 600 to 1,600 fathoms. The eyes of all the deep-sea species are relatively larger than those of the shallow-water ones, except _Serolis gracilis_, whose eyes seem to be disappearing. But these large eyes of the deep-sea species of _Serolis_ are not capable of any greater perceptive power. In fact, the evidence of degeneration they show, both in minute structure and in the diminution of pigment, proves that they can be of very little use to these animals for perception (see Figs. 4 and 5). This increase in size, accompanied by degeneration of structure, is just what we should expect to find in the eyes of deep-sea animals, and it is difficult to explain why it is that we do not find more examples of it. If the animals that now live in the depths of the sea are descended from the shallow-water forms of bygone epochs, they must have passed through many different habitats with diminished light until they reached their present dark abode in the abyss. [Illustration: FIG. 4.—Semi-diagrammatic section through the eye of _Serolis schythei_, a shallow-water species (4–70 fathoms). C, lens; V, crystalline cone; R, rhabdom; N, nerve. (After Beddard.)]