It is in the deep sea also that we find the curious Hermit Crabs of the family Pylochelidæ ([Fig. 37], p. 94), which are perfectly symmetrical and show no trace of having ever adopted the habit of living in Gastropod shells; so primitive, indeed, are these forms that it is not easy to find characters by which to define them from the lobster-like Thalassinidea or from the true Lobsters themselves, and, although no fossil representatives are yet known, there seems no reason to doubt that the Pylochelidæ are nearly related to the primitive stock from which the other Hermit Crabs have been evolved. Among the deep-sea Prawns there are many forms, both of Penæidea and of Caridea, which are more primitive than most of their relatives from shallow water; and although in these cases also the geological records are faulty, we may assume, if we cannot prove in detail, a general similarity to the fossil Prawns of Mesozoic rocks.
When all has been said, however, perhaps the most surprising thing about the deep-sea fauna is, not that the animals are unlike those living in shallow water, but that they differ from them so little. When we consider the physical conditions of the oceanic abysses—the absolute darkness, the freezing cold, the pressure measured in tons on the square inch—it would seem inevitable that the physiological processes of deep-sea animals must differ greatly from those of animals living in shallow water; yet in very many cases these differences of function are accompanied only by the most trivial differences in structure. To take one example, the "Pink Shrimp" (Pandalus montagui), which we may find commonly between tide-marks on our own coasts, differs only in inconspicuous details from species of the same genus living at a depth of 600 fathoms; while other genera of the family Pandalidæ range downwards to 2,000 fathoms or more, without any important divergences in structure.
[CHAPTER VII]
FLOATING CRUSTACEA OF THE OPEN SEA
It is only rarely that the floating organisms of the surface of the sea are so large or so abundant as to catch the attention of the casual observer. Except for an occasional shoal of porpoises or of flying-fish, the waste of waters seen from the deck of a ship in mid-ocean usually seems to be barren of life. Nevertheless, there is probably no region of the ocean where the tow-net will not reveal the existence of a more or less varied fauna and flora. Sometimes, indeed, these organisms, though minute, are so numerous as to discolour the water over large areas; whalers in the Arctic seas know by the appearance of "whale-food" where whales are likely to be found, and Herring or Mackerel fishermen recognize the changes in colour of the water among the "signs" which guide them when and where to shoot their nets.
The organisms which make up this "pelagic" fauna and flora may be grouped into two classes, which may be termed the "swimmers," or Necton, and the "drifters," or Plankton. The former include the larger and more active animals, such as fish, whales, and the like, whose movements are more or less independent of the movements of the water; the latter comprise the plant-life and the floating or feebly swimming animals that drift at the mercy of waves and currents. A great deal of attention has been given in recent years to the study of the plankton, and it has come to be recognized as filling a very important place in the balance of life in the sea. In the sea, as on land, all the animals are ultimately dependent on plants for their food. The larger and more conspicuous sea-weeds which grow on the sea-bottom, however, can only flourish in comparatively shallow water, and the region which they occupy forms only a narrow fringe round the land-masses of the globe. It is only necessary to look at a map of the world, showing the depth of the sea, to realize what an insignificant part of the area of the oceans contributes in this way to the food-supply of marine animals. The microscopic plant-life of the plankton, however, makes up for the individual minuteness of its constituents by their incalculable numbers. The lowly organisms known as "diatoms," familiar to the microscopist from the beauty of their flinty skeletons, are among the most numerous and important of these, and they are associated with a great variety of other single-celled algæ and allied organisms, some of them so minute that they pass through the finest silk plankton-nets, and have to be sought for by special methods of collection recently devised for the purpose. All these organisms possess the green colouring matter (chlorophyll) that enables them to live, as the higher plants do, on the carbon dioxide and other substances dissolved in the water. The smaller animals of the plankton feed on these vegetable organisms, and in their turn serve as food for larger animals. The Herring, the Mackerel, the gigantic Basking Shark, and the still more gigantic Greenland Whale, all feed directly on the animal plankton, and we have already seen that the animals of the deep sea depend entirely on the same source of food-supply. Further, very many of the bottom-living animals of shallow water swim at the surface in the early stages of their life, and feed on the other plankton animals and plants. Indeed, it is no exaggeration to say that "all fish is diatom" in the same physiological sense as "all flesh is grass," and the study of the plankton is thus of practical importance as well as of scientific interest.
Of all the minute animals that form the intermediate links in the chain between diatom and fish or whale, the Crustacea are the most important and the most numerous both in species and in individuals. The Copepoda are more richly represented than any of the other groups, and it would be difficult to find a sample of marine plankton from which they were altogether absent. Associated with them we find one or two species of Cladocera, a larger number of Ostracoda (chiefly of the family Halocypridæ), a few Mysidacea, the Amphipoda of the suborder Hyperiidea, the Euphausiacea, and some of the shrimp-like Decapods; while the larval stages of these and other groups also form an important part of the plankton.
It is necessary to make a distinction between the "neritic" plankton of shallow water near the coast and the "oceanic" plankton of the open sea. In the inshore waters the plankton consists not only of organisms that pass the whole of their life at or near the surface, but also, and very largely, of the free-swimming larvæ of bottom-living species, and of others that make occasional and temporary excursions to the surface. For example, if the tow-net be used a short distance from land—say in some sheltered bay on our own coasts—the catch will often be found to consist largely of larval Crustacea. The zoëa and megalopa stages of Crabs, the zoëa and schizopod stages of Prawns and Shrimps, are often conspicuous by their numbers, or we may find swarms of the nauplius and cypris larvæ of Barnacles. Sometimes, and especially at night, numbers of Cumacea may be found in the tow-net; and it is noteworthy that these are usually males, which leave the females burrowing in the mud at the bottom, and swarm to the surface for a brief period of activity. Besides all these more or less temporary visitors, however, there are numerous species, even in the inshore waters, which are adapted to a floating life, and pass their whole existence as members of the plankton. Copepoda of many kinds, some Mysidæ, Amphipods like Hyperia—which is commonly found sheltering under large jellyfish—some species of actively swimming Isopods, and many other forms, are only to be captured by the tow-net; and now and then, in certain localities, winds and currents may drive into coastal waters shoals of species whose proper home is the open ocean.