having the mouth opening at its end, instead of a nostril), which hangs from the centre of the disk-like jelly-fish.[2] [2] See "Science from an Easy Chair" (First Series, 1910), p. 60, for a further account and figure of the freshwater jelly-fish. There are some glass-like transparent creatures, including some small fishes, which live at 500 fathoms depth and a good deal deeper on the sea bottom. We know that the sun's light does not penetrate below 200 fathoms, so that one is led to ask--What is the good of being transparent if you live at the bottom of the sea, at a greater depth than this? There is also a very beautiful prawn, which I dredged in Norway in 200 fathoms, which looks like a solid piece of clearest, colourless glass. And then there are some very beautiful little stalked creatures (called Clavellina), fixed to the under-side of rocks in the tidal zone, which are absolutely like drops of solid glass an inch long. One cannot easily imagine how colourless transparency can be of "life-saving value" to these varied inhabitants of the dark places of the sea bottom--any more than we can assign any life-saving value to the brilliant, gem-like colouring of some of the sea-anemones which live in the dark on the under-surface of rocks. The most probable view of the matter is that neither the colourless transparency of the one set nor the brilliant colouring of the other has any value; it just happens to be so, and is not harmful. So, for instance, some crystals are colourless, some blue or green or yellow or red, without any advantage to them! On the other hand, we know that a large number of the animals which live in the dark unfathomed depths themselves produce light, that is to say, are phosphorescent, and it seems probable that at great depths, though there is no sunlight, the sea bottom is illuminated--we can only vaguely guess to what degree--by the strange living lanterns--fish, crustaceans, worms, and even microscopic creatures--which move about in quest of their food, carrying their own searchlight with them. Another suggestion is that the eyes of these inhabitants of the dark may be more sensitive than our own, and even be affected by rays invisible to us. This, however, is not probable, since whilst there are among them some with enormous eyes, we find that at the greatest depths (2 to 4 miles) even the fishes have no eyes at all, and at a depth of a mile there are many shrimp-like creatures in which the eyes have been completely transformed into peculiar "feelers," or otherwise aborted. So that we cannot suppose there is a possibility of developing the eye of the dwellers in deep-sea darkness to a degree of sensitiveness greatly beyond that of terrestrial animals. A limit of obscurity is reached at which it is of no use having an eye at all, and eyes cease to have life-saving value, and accordingly are not maintained by natural selection.