The second includes all animals which live between about three hundred fathoms of the sea-bottom and three hundred fathoms of the ocean-surface.
The third includes all animals which live within three hundred fathoms of the surface, and those on shallow shores.
By far the greater number of living creatures appear to inhabit the surface regions; and it may be that the next in number are those on and near the ocean-bed. This, however, is uncertain. It is difficult to judge about the intermediate depths. All creatures living there must be extremely good swimmers, and they are very shy of nets.
We cannot readily picture to ourselves the manner of life which goes on in those intermediate parts. Creatures there have, as it seems, no connection with either the ocean-bed or with shallow shores—no resting-place to which they may turn—in short, no home. Though one may not realise the fact, few animals within our ken have not something in the shape of a home, a pied à terre, to use a familiar term, though that “terre” may be under water.
The very idea of a life spent always in swimming or floating through boundless depths, never touching firm ground, having no kind of home or retreat, suggests vagabondage, and sounds disconsolate.
It may be that the difficulty is at least partly met by those great floating banks of living creatures which are often found.
Diatom-Banks have been already spoken of. But other animals of larger growth also band together, forming vast companies. To each individual in such a congeries the Floating Bank would be a home. It would matter little to that individual whether the bank as a whole floated here or drifted there, whether it rose at night to the surface of the sea, or whether in daytime it sank lower to escape from the sun’s glare.
Many such banks of life belong strictly to the Surface Class rather than to the Intermediate. But the want of a home applies equally to all creatures who live a free and roving life in the Ocean, unattached to shore or to sea-bed, whether they live above or below the three-hundred fathom limit.
Another puzzling question has been as to how deep-sea creatures are fed.
In Life on terra firma, generally, the work of plants is to make ready food for animals. Many substances, which are needful for animal-structure, cannot be taken in by them until broken up and re-made by plants. And plants can only carry out this task in sunlight. Where the light of the sun fails, there plant-life fails also.