The eyes of the animals that live in deep-sea water undergo curious modifications. If the fauna of the abysmal region were confined to conditions of absolute darkness, we should expect to find either a total absence of eyes or mere rudiments of them only in those forms that have recently migrated from the shallow water. This is the case with the fauna of the great caves. There is probably total darkness in these underground lakes and streams, and there is only the remotest possibility of the animals living in them ever seeing, even temporarily, a ray of sunlight or even a glimmer of phosphorescence during the whole of their life-time. We find then that the cave fauna is totally blind. The conditions in the deep sea are not quite the same. In some regions there is probably a very considerable illumination by phosphorescent light, and it is quite possible that many of the characteristic deep-sea forms may occasionally wander into shallower regions where faint rays of sunlight penetrate, or even that the young stages of some species may be passed at or near the surface of the sea. Taking these points into consideration, then, it is not surprising to find that, in the deep seas, there are very few animals, belonging to families usually provided with eyes, that are quite blind. In the majority of cases we find that the eyes are either very large or very small. Only in a small minority of cases do we find that the eyes are recorded to be moderate in size. The relation between the large-eyed forms and the small-eyed forms is not the same in all the regions of deep seas. In depths of 300 to 600 fathoms the majority are large-eyed forms. This is as we should expect, for it is more than probable that many of these forms occasionally wander into shallower waters where there is a certain amount of sunlight. In depths of over 1,000 fathoms, the small-eyed and blind forms are in a majority, although many large-eyed forms are to be found. Among fishes, for example, we find the species of _Haloporphyrus_ found in depths of 300–600 fathoms with large eyes; and so with _Dicrolene_, _Cyttus abbreviatus_, and many other forms that are known to live in water of less depth than 700 fathoms; while on the other hand in _Melanocetus Murrayi_, _Ipnops Murrayi_, many deep-sea eels and other fish that are truly abysmal and live chiefly in depths of over 1,000 fathoms, the eyes are either very small or absent. Some interesting examples may be found in the species of widely distributed genera to illustrate these points. Thus in _Neobythites grandis_, from 1,875 fathoms, the eye is small, only one-eleventh the length of the head, but in _Neobythites macrops_, _N. ocellatus_, and _N. gillii_ from shallower water it is much larger.