In the first place bathybial fish, mollusca, crustacea, and other animals usually possess a remarkably small amount of lime in their bones and shells.
In fishes we are told that the bones have a fibrous, fissured, and cavernous texture, are light, with scarcely any calcareous matter, so that the point of a fine needle will readily penetrate them without breaking. In some the primordial cartilage is persistent in a degree rarely met with in surface fishes, and the membrane bones remain more or less membranous or are reduced in extent, like the operculum, which is frequently too small to cover the gills.
This cannot be due in all cases to a deficiency of carbonate of lime in the sea water, for we find these characters well marked in some of the fish, such as Melanocetus Murrayi, Chiasmodus niger, and Osmodus Lowii, that are found on the Globigerina mud.
Then again, the shells of the deep-sea Lamellibranchs, Gasteropods, Brachiopods, and Crustacea are very frequently remarkably thin and transparent, a character that is probably more generally due to a weakness in absorptive or secretive activity than to a deficiency in the supply of lime.
There are one or two characters of the deep-sea fish that it is not easy to account for, and it is necessary only to mention their occurrence without attempting to offer any explanation of them.
One of the most common of these is the very dark pigment occurring in certain parts of the epithelium of the mouth and respiratory passages and the endothelium of the peritoneum. For example, in Bathysaurus mollis, living at a depth of 2,000 fathoms, the mouth and buccal cavities are black. The same thing occurs in Ipnops Murrayi, and indeed in all the strictly deep-sea forms.
Another important character of very frequent occurrence is the reduction in size, length, and number of the gill laminæ.
Among invertebrates we may mention as a fact of some interest, dependent perhaps on the soft character of the bottom, the preponderance of stalked forms over those of more sessile habits.
Thus among the Alcyonaria the characteristic forms of the deep water are the Pennatulids, and more particularly the genus Umbellula with its long graceful stem and terminal tuft of polyps. Among the Echinoderma we find many forms of stalked Crinoids. Among the Tunicates several curious genera characterised by their long peduncles.
Taking the fauna as a whole, Moseley regarded it as similar in some respects to the flora of the high mountains. Some forms are dwarfed in size, such as the species of Radiolaria, Cerianthus, some of the Cephalopods, &c., while others are very much larger than their shallow-water allies, such as the Pycnogonids, nearly all the Crustacea, Alcyonarians (as regards the size of the polypes), Siphonophora, and many others.