Similarly in the species of the widely distributed deep-sea genus Macrurus: the species M. parallelus, M. japonicus, M. fasciatus, &c., usually living in water less than 1,000 fathoms deep, have large and in some cases very large (M. fasciatus) eyes, but Macrurus filicauda, M. fernandezianus, M. liocephalus, M. Murrayi, M. armatus have small eyes.
Some deep-sea fish have their eyes reduced to a mere rudiment; such as Ceratias uranoscopus, C. carunculatus, Melanocetus Murrayi, Typhlonus nasus, and Aphyonus gelatinosus, but not even a rudiment of an eye is to be found in Ipnops Murrayi.
But the fish of the greatest depths are by no means always characterised by small eyes. Malacosteus, a typical deep-sea form, has very large eyes, and so have Bathylagus, living in the enormous depth of 3,000 fathoms, and Bathytroctes, in 1,090 and 2,150 fathoms.
The result of recent deep-sea work, then, has been to show that as we proceed from shallow shore water to depths of 500 to 900 fathoms the eyes of the fish become larger, but in greater depths than 1,000 fathoms the eyes of some fish become considerably reduced, but those of others become still more enlarged. In the greatest depths of the ocean in fact it seems very probable that nearly all the fish are characterised by either very large eyes or very small ones.
We cannot expect to learn very much at present from the study of the eyes of deep-sea mollusca. The Cephalopods form the only class of this Phylum whose genera invariably possess large and well-developed eyes, and there does not seem to be any very marked increase or decrease in the size of the eyes of the few deep-sea cuttlefish that are known to us.
The eye of Nautilus is certainly remarkably interesting, but as this genus is the only representative of its order, and is known at times to float upon the surface of the ocean, it would certainly be erroneous to attribute the peculiarity of the structure of its eye to its ‘temporary’ deep-sea habits. We are still ignorant of the usual habitat of the remarkable genus Spirula, notwithstanding the fact that many of the tropical beaches are very largely composed of its empty shells. Whether it is a deep-sea dweller or not, we know nothing at present of the character of its eye, so that it can throw no light upon the problems we are now discussing.
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.