It has already been mentioned that the animals living on the sea-bottom in shallow water do not differ greatly in character from those that may be found between tide-marks. As we go farther out from land, however, into the deeper water, the character of the fauna gradually changes. One by one the species found near the shore become rare and disappear, and their places are taken by others characteristic of the intermediate depths. These in their turn give way to others, till in the abysses of the great oceans we find an assemblage of strange animals adapted to the conditions of life in the great depths, and differing widely in many respects from the more familiar inhabitants of the coastal waters. In this "fauna of the deep sea," which extends to the greatest depths reached by the dredge or trawl, the Crustacea occupy a prominent place. Before proceeding to discuss some of these peculiar forms, however, it is necessary to attempt to form some idea of the conditions under which they live. In the first place, the character of the sea-bottom changes very greatly as we pass away from the coast. Near the shore it is extremely diversified, consisting in one place of rocks swept bare by the tides or overgrown with jungles of sea-weed, in another of banks of gravel or shingle, of sand or of mud, but in all cases derived from the "waste" of the land, as it is eaten away by the waves or washed down by the rivers. As the distance from land increases, the deposits become finer and finer, till they shade off into a soft oozy mud, composed of the finest particles brought down by the rivers. In the neighbourhood of large rivers this mud may sometimes extend for hundreds of miles from the land, but there is a limit to the distance to which even the finest particles can drift before they settle to the bottom, and beyond this limit the floor of the ocean is covered by sediments which owe their origin, not to the land, but to the ocean itself. The surface waters of the ocean everywhere teem with a vast variety of floating animals and plants, and, as these die, their remains sink to the bottom "like a perpetual shower of rain." Among the most abundant floating organisms--in the warmer seas, at any rate--are certain minute animals known as _Foraminifera_, which belong to the lowest class of the animal kingdom, and have shells composed, in most cases, of carbonate of lime. Over vast areas the bottom of the ocean is covered with a soft grey ooze, made up almost entirely of the dead shells of Foraminifera rained down from above. Since the commonest species of Foraminifera found under these circumstances belong to the genus _Globigerina_, the deposit is known as "Globigerina ooze."