It is very probable, however, that these currents at the bottom of the ocean are extremely slow, and, as the water is never affected by tides or storms, the general character of the deep sea is probably one of calm repose. This is a matter of no little importance; for, in the consideration of the characters presented by the fauna of any particular region, it is always necessary to take into account the physical difficulties the animals have to contend against and the modifications of structure they present to combat these difficulties. Thus in a region such as that presented by the deep sea, where there are no rapid tides, we should not expect to find such a powerful set of body muscles in the free-swimming forms nor such a firm vertebral column as in the animals that live in more lively water. Perhaps it is of the nature of an assumption to say that there are no rapid currents and tides in the abysmal depths of the ocean, for we have no means of demonstrating or even of calculating the rate of flow of these waters. But it is a reasonable hypothesis and one that we may well use until the contrary is proved. A fact of some importance that supports this hypothesis, as regards some parts of the ocean at least, is presented by the sea-anemones. Many of the shallow-water Actinians are known to possess minute slits in the tentacles and disc, affording a free communication between the general body cavity or cœlenteron and the exterior. In many deep-sea forms the tentacles are considerably shorter and the apertures larger than they are in shallow-water forms. It is difficult to believe that such forms, perforated by, comparatively speaking, large holes, could manage to live in rapidly flowing water, for if they did so they would soon be smothered by the fine mud that composes the floor of all the deep seas. In fact anemones of the type presented by such forms as _Sicyonis crassa_ are only fitted for existence in sluggish or still water. [Illustration: FIG. 2.—_Sicyonis crassa._ M, mouth; S, ciliated groove; T, tentacles. Each tentacle is perforated by a single large aperture. (After Hertwig.)] Another character that must be taken into consideration is that presented by the floor of the great oceans. The floor of the ocean, if it were laid bare, would probably present a vast undulating plain of fine mud. Not a rock, not even a stone would be visible for miles. The mud varies in different parts of the globe according to the depth, the proximity to land, the presence of neighbouring volcanoes or the mouths of great rivers.