Nearly all the animals at extreme depths--practically all the animals, for the small number of higher forms feed upon these--belong to one sub-kingdom, the Protozoa; whose distinctive character is that they have no special organs of nutrition, but absorb nourishment through the whole surface of their jelly-like bodies. Most of these animals secrete exquisitely formed skeletons, some of silica, some of carbonate of lime. There is no doubt that they extract both these substances from the sea-water; and it seems more than probable that the organic matter which forms their soft parts is derived from the same source. It is thus quite intelligible that a world of animals may live in these dark abysses, but it is a necessary condition that they must chiefly belong to a class capable of being supported by absorption through the surface of their bodies of matter in solution, developing but little heat, and incurring a very small amount of waste by any manifestation of vital activity. According to this view it seems probable that at all periods of the earth’s history, some form of the Protozoa--rhizopods, sponges, or both--predominated greatly over all other forms of animal life in the depths of the warmer regions of the sea. The rhizopods, like the corals of a shallower zone, form huge accumulations of carbonate of lime, and it is probably to their agency that we must refer most of those great bands of limestone which have resisted time and change, and come in here and there with their rich imbedded lettering to mark like milestones the progress of the passing ages. We find the first and simplest of the invertebrate sub-kingdoms, the Protozoa, represented by three of its classes--the monera, the rhizopoda, and the sponges. The monera have been defined as a distinct class by Professor Ernst Haeckel, of a vast assemblage of almost formless beings apparently absolutely devoid of internal structure, and consisting simply of living and moving expansions of jelly-like protoplasm. The monera pass into the rhizopoda, which give a slight indication of advance in the definite form of the graceful, calcareous, shell-like structures which most of them secrete, and the two groups may be taken together. The dredging at 2,435 fathoms at the mouth of the Bay of Biscay gave a very fair idea of the condition of the bottom of the sea over an enormous area, as we know from many observations which have now been made with the various sounding instruments contrived to bring up a sample of the bottom. Under the microscope the surface-layer was found to consist chiefly of entire shells of Globigerina bulloides, large and small, and fragments of such shells mixed with a quantity of amorphous calcareous matter in fine particles, a little fine sand, and many spicules, portions of spicules, and shells of Radiolaria, a few spicules of sponges, and a few frustules of diatoms.