In the study of the marine fauna and flora we must notice, it is the depth of the water, or in other words the depression of the habitats below the sea-level, that forms the most important consideration. Geographical sub-regions may be recognised and defined with a certain amount of accuracy, especially in the case of the fauna of the shallow waters, but by far the most important changes in the general characters of the fauna are found when we pass from one ‘zone’ of depression to another. Thus in describing any particular marine fauna we should mention first of all its zone or sub-zone of depression and then its geographical region and sub-region. For example, we may speak of the fauna of the pelagic zone of the British sub-region of the European region, or the fauna of the abysmal zone of the Northern sub-region of the Atlantic region. We can recognise three primary zones of the marine fauna which we may call the ‘Pelagic,’ the ‘Neritic,’ and the ‘Abysmal’ zones. The Pelagic zone includes the superficial waters of all seas extending from the surface to a depth which cannot at present be very accurately determined, but is probably the same as the limit of the influence of direct sunlight. The animals of this zone are frequently characterised by a general transparency of their tissues, a white or sea water (i.e. blue or green) colour, an organisation capable of prolonged swimming or floating movement, and by giving birth to floating eggs which hatch out transparent larvæ or embryos. The pelagic zone may be divided into several geographical regions and sub-regions, which it would be beyond the scope of this book to enumerate here, but there is one that calls for a few brief remarks. In many parts of the ocean there may be found vast areas of floating sea-weed, which carry with them a population of crustacea and other animals peculiarly their own. This ‘sargasso’ fauna presents so many characteristics and so many features different from that of the ordinary pelagic fauna, that the tracts of sea bearing this weed must be considered to rank as a special region of the pelagic zone, which may be called the Sargasso region. The zone of shallow water for which we shall adopt Professor Haeckel’s term—the Neritic zone—embraces all parts of the seas of less depth than 500 fathoms, including the inland seas, the shores of great continents and islands, and the shallow banks in the great oceans. It does not include the superficial waters—which belong to the pelagic zone—but extends only from the actual bottom to a distance of a few fathoms above it. The fauna of this zone is extremely varied, consisting of animals that swim, crawl, or are permanently fixed to the bottom, animals of almost every variety of colour and marking, and of every size and shape.