Through these dark abysses swim fishes with extraordinary and grotesque adaptations to their conditions. All are small, rarely six inches long, often less than an inch, yet armed to the teeth. This is especially true of the families Stomiatidæ and Sternoptychidæ, in which one finds fishes of the queerest shape, with big heads and a savage array of long sharp teeth. All are voracious, for food is scant and must be fought for; and some, as Chiasmodus, have mouths so capacious that they often swallow fishes larger than themselves, when their stretched stomachs hang beneath their slender bodies like the yolk sacs of newly born trout. All are dark in color, brown, blue or violet marking the abyssal species. Some of them have light-giving organs; and this was formerly regarded as a peculiar possession of deep-sea fishes, enabling them to see their prey in the gloom of their habitat, but it is now known that light-giving organs are especially characteristic of pelagic fishes of the region between the surface and 250 fathoms of depth. It must be remembered, however, that the sedentary invertebrates of the bottom glow with phosphorescence. This outline of a vast body of information shows that the waters of the oceans are everywhere inhabited, to their uttermost deeps, by living beings; that these are adapted to various circumstances, and so form faunas of local extent and character; and that probably the sea derived its wealth of population--at least all that part superior to the monads--from the land, beginning with the earliest dawn of life on the globe. CHAPTER III A CHAPTER OF FOUNDATIONS THE NATURAL BASIS OF CLASSIFICATION I mentioned in my introductory chapter that the simplest form of animal was one whose whole being was contained within a single envelope, or "skin," called a cell. Such a cell contains nothing but that strange primitive life-substance named protoplasm, condensed at one point into a nucleus, and it is precisely of such cells that the bodies of all the animals we commonly know are made up; nevertheless an immense variety of creatures still exists, especially in the plankton of the sea, that, like those at the dawn of life, consist of one cell alone. Here then we stand at the first grand division of the animal kingdom: A. Animals consisting of a single cell--_Protozoa_. B. Animals composed of an aggregation of cells--_Metazoa_.