However, there is little doubt that there are some truly abysmal Protozoa. Among the Radiolaria, for example, it seems extremely probable that the majority of the Phæodaria and many Spumellaria live only in very deep water. ‘A character common to these abyssal forms,’ says Haeckel, ‘and not found in those from the surface or slight depths, is found in their small size and massive heavy skeletons, in which respects they strikingly resemble the fossil Radiolaria of Barbadoes and Nicobar islands.’ The Phæodaria are very widely distributed over the floor of the ocean, and occur in some districts in such numbers that the ‘Challenger’ was able to bring home some hundreds of thousands of specimens. They are distinguished from other Radiolaria by the thick outer and thin inner capsule, by the typical main opening or atropyle placed on the oral pole of the main axis with a radiate operculum provided with a tubular proboscis, and lastly by the presence of the phæodium, a voluminous pigment body which lies invariably on the oral half of the calymma and is composed of numerous singular pigment granules of green, olive, brown, or black colour. [Illustration: FIG. 9.—_Challengeria Murrayi_, one of the Phæodaria (2,250 fathoms). A, phæodium; B, central capsule; C, strands of protoplasm in the calymma. After Haeckel.] There are many genera belonging to the Foraminifera that are very probably inhabitants of abysmal depths, but they do not seem to possess any special characters, unless it be a greater thickness and density of their shells, to distinguish them from their shallow-water allies. Passing now to the group of the sponges or Porifera, we find that the calcareous sponges are not represented at all in the abysmal zone. Two species are found at a depth of 450 fathoms, but none are truly bathybial in habit. The same remark applies to the horny sponges. These forms chiefly belong to the littoral or very shallow-water fauna, and never descend to greater depths than 400 fathoms. Of the other groups of Porifera—the Monaxonia, the Tetractinellidæ, and the Hexactinellidæ—several genera are known to extend down to some of the greatest depths at which trawling operations have been successfully carried on. It is difficult to point to any characters in these sponges that can be attributed in any way to the conditions of deep-sea life, but nevertheless we do find in deep water some of the most remarkable and beautiful forms of sponge skeleton that can be found anywhere. Amongst the Cœlentera we find in the deep water a remarkable sub-family of Medusæ, which has been named by Haeckel the Pectyllidæ. It is distinguished from the other jelly-fish by the curious sucking cups situated at the ends of the tentacles. It seems probable that they are used for purposes of locomotion, the animal walking over the muddy bottom as on a series of stilts.