In this plankton are multitudes of minute protozoans (q.v.), principally foraminifers and radiolarians in a profusion of species. They have shells consisting either of lime or of silica, and it is these shells, raining down upon the bottom, that constitute the globigerina and other oozes. Here, too, float a great variety of salpse, jellyfish and other coelenterates and their larva, and many swimming worms, notably the wide spread sagittas. Outnumbering all these, except the Protozoa, are the oceanic crustaceans, which, in countless forms and multitudes, play there, as Ha-ckel remarks, a part corresponding to the insects on land. Most numerous of these are the copepods, all very small ; and they are the chief consumers of the minute plants, and, in turn, are among the most important food supplies for larger creatures. Almost equally numerous are the ostracods and other crusta ceans, including some big crabs.
Mollusks are fewer, for these as a rule are bottom-dwellers; but one group, the delicate, winged pteropods, are so extraordinarily numer ous, that their glossy shells characterize large patches of ooze in the tropics. One species is called (whale's food,D and actually is the princi pal subsistence of the northern whalebone whales. A large variety of small squids is also to be found at or near the surface of the open ocean.
The pelagic plankton, which is very largely different from that of coastal regions, is aug mented in summer by an extensive variety and amount of larva and young animals born both there and in the deeper layers; and at night many animals rise to the surface that are never caught there in daytime. Finally, it is the abode of innumerable fishes that seek their prey at or near the surface. Among these the family Scope!idle, although represented also in the abyssal fauna, play a more important part at the surface than all other fishes combined; yet most of them are only a fraction of an inch long. Transparency and colorlessness (or a blue tint) are characteristic (for safety) of all the lesser creatures in this surface-fauna.
Intermediate Depth-Zones.—It has been said already that between the surface and the bottom the sea seems to be divided into layers or strata in respect to its inhabitants, certain kinds of animals when adult dwelling only within certain limits of depth. These inter mediate zones, and their occupants are called a term including the facts of both their undersea and deep-water existence. Practically, however, these intermediate zones can hardly be defined, and seem to be determined by limits of temperature or of osmotic pressure, and thus vary in their distance below the sur face according to the general warmth and den sity of various regions. Animals taken only by deep hauls of the nets within the tropics, for instance, may be found near the surface in cooler latitudes; furthermore the vertical dis tribution of fishes, as a class, may differ from that of crustaceans or other groups as a class.
Nevertheless it is generally true that many sorts of pelagic animals dwell at intermediate depths, from which, when adult, they cannot either rise or descend to any great distance. Space cannot be given to much evidence of this, but the fol lowing extract from the report of the San Expedition* (1910) gives the following general results of its dredgings and net-haul :tags, as bearing on this point, as follows: (These catches may be classified into three main regions: (1) A region extending down ward from about 500 metres, characterized by the recurrence of Cyclothone and various black or dark-colored fishes, and of many peculiar inver tebrates, red prawns being prominent; (2) a region ranging between 150 and 500 metres, characterized by a peculiar community of sil very or grayish fishes belonging to the families Sternoptychide and Stonnatider; and (3) the surface region, comprising the upper 150 metres. . . . The layer from 800 to 1,000 metres downward may require to be still further sub divided, for certain forms, like the larger Acanthophyra with red eggs, Notostomus, and several fishes and squids have been taken only in the deepest hauls at 1,500 or 2,000 metres.' Abyssal Life.—Let us now consider the creatures of the abyssal depths, where eternal cold, stillness, darkness and equability unite to make an environment so forbidding that the imagination would refuse to people it with living beings, yet where life and strife do actually abound. This bottom fauna, however, is not equally distributed. It is far more scanty on the areas of red clay than on the ooze; and the fauna of the various ocean-beds differ, because barrier ridges separate them. What is said here relates to that of the bed of the deep Atlantic, which is better known than any other.
The real bottom-animals are mainly, fixed, and consist of sponges, hydroids, actinians, bryozoans, ophiurans, crinoids, brachiopods, holothurians, worms and mollusks. These are nowhere numerous, and below 2,500 fathoms are very scarce, to judge by the results of dredg ing. Their subsistence is wholly derived, appar ently, from the surface, some catching it as it falls and others sucking it out of the top layer of the ooze. Moving about there also is a lim ited population of snails, crabs, squids and fishes, making their living upon or close to the bottom; and a larger and more varied company of their relatives swim in the water above them, up to, say, the Z000-fathom line. All are of forms different in many respects from related species at or near the surface, and some brought up by the dredge can hardly be distinguished from fossils entombed in the oldest fossiliferous rocks, so unchangeable is the environment in which their race has been propagated for per haps 50,000,000 years.