As a rule, the light organ is surrounded by a layer of black pigment that acts as a reflector, and often the light is projected through a transparent lens; and there is reason to believe that in the case of the higher animals, such as deep-sea fishes and squids, the rays may be thrown when and where the creature desires, as a man handles an electric flashlight. But for what purpose? Is it to illuminate the surrounding water so as to perceive, or to attract prey, or is it to avoid foes? A learned oceanographer replies that no one certainly knows. "At all events," he concludes, "the answers would probably tend to show that the many different kinds of light organs serve different purposes." PRISONERS IN THE DARK AND ICY DEPTHS So much for the surface population of the ocean--the plankton layer is regarded as a hundred fathoms thick. We have considered only that over the mid-oceanic depths, but that of the shallow margins is different simply in the absence of some purely pelagic creatures, and in the presence of vast hordes of eggs and larvæ of the animals rooted in the sand or attached to the rocks and weeds from high-water mark down to a comparatively short distance below low-water mark. These I shall speak of more completely hereafter. Before that, however, I want to say a few words in regard to the extraordinary inhabitants of the ocean's depths--depths which in some places exceed the elevation of the highest mountains on the land. The conditions under which animal life exists there are vastly different from those at the surface, and it is not surprising to find these creatures of an extraordinary character. The pressure exerted by water on anything lowered into it increases at a rapid rate as the object sinks, so that at a depth of only 500 fathoms it equals about 100 times the pressure at the surface. This contributes to the density of underlying waters; the saltiness of the sea also adds to the water's density, but this decreases slightly from the surface downward. More important than density in its effect on living things is temperature. In the Sargasso Sea in summer the water at the surface will indicate about 52 degrees F., and at 100 fathoms of depth 48 degrees, below which it diminishes slowly to a little below the freezing point--32 degrees F. The water below a few hundred fathoms may therefore be regarded as a series of layers measured by degrees of density, temperature, etc., and this means a series of biological strata in each of which the denizens are more or less limited by unfavorable conditions above and below them.