WITHIN THE SEA. 1 tela ted to climatic influences on land are those conditions its the ,ea which set invisible bounds to the spread of most marine animals, even when ap parently at full liberty to go anywhere. A few species of big whales. simrks. and predatory fishes are known in all parts of the oceanic world, and even individuals are often very far ranging. Some others are spread throughout all the northern seas. or all the southern, as the ease may he: but of tropical marble animals very few are com111011 to both the Atlantic and the Pacific. On the contrary, the distribution of marine animals of every sort exhibits local re striction as fully as does that of land animals. Alarine animals may be divided into three classes a. relates to the present theme, namely the Lit toral Fauna, Pelagic Fauna, and Abyssal Fauna. To the first belongs the crowded life of the shore region, where the rocks and forests of kelp, coral reefs, awl natural bottom from high-water mark down to ltni fathoms or so of depth, are the home of a vast number and variety of fixed and mov iwr creatures of the lower orders. and a host of higher arthropods. fishes. and cetaceans which live among. or upon them. These vary with the They are most abundant in the tropics and decrease toward the poles. Drifting sand and mud are nearly barren, but rocky shores are populous. Currents of cold water affect a coast unfavorably, while warm currents bring and sustain many species. Thus physieal and climatic conditions influence the amount and dis tribution of shore life beneath the water much as they do that above it.
The 'pelagic' fauna includes those animals which habitually dwell upon or near the surface of the open ocean, visiting all parts of it. They may be able to swim actively and so move at will here or there, and such form the class 'nek ton,' or. like the jellyfishes and snips. may only float and be drifted about by winds and currents (the class 'plankton'). Even here. however. cer tain specie; and groups are to he gathered only in certain parts of the sea: and their range seems to be limited mainly by factors of temperature. Thus the fauna of the Gulf Stream is distinct from that of the Atlantic for a considerable dis tance north of Florida.
11AutvmErnie DISTRIBUTION. A new element
enter- into the question of the distribution of life in the sea, namely. variation in depth. This is comparable, in reverse. to hypsometrie distribu tion. or that according to height above sea-level. Layers of animal life, as it were, may be ob served from the shore line to the greatest ex plored depths. Most of the ereatures to be found tide-marks are absent or rapidly decrease below a few fathoms, while ninny rarely the shore, but are numerous on bot toms covered by 100 to 300 of water. Another zone belongs mainly or exclusively be low that ; and the globigerina ooze (q.v.), cover ing the ocean bottom with grayish mud in most part, of the world, from 400 fathom: down to about f?onn fathoms. has a distinct fauna of its own. Below 2500 fathoms the sea-bottom is formed of red clay, in Which shells are b sent.
having apparently been dissolved during their descent to the greater depth. Even here. how ever, is found an abyssal fauna chiefly of fishes, "often of a very grotesque appearance." This bathym•trie distribution. from the 100 fathom line down, depends upon the factors of temperature and density. The former would be influeneed by the ocean currents, and the animal life in the path of an influx of water from the polar region, would be different from that in the path of a warm current. The abyssal fauna is one habituated to such a degree of cold as would instantly kill much of the littoral or surface fauna. In addition to this powerful localizing influence, that of density. increasing with depth of water, is supreme as limiting the upward and downward range of animals habituated to a certain stratum—that is, to a certain average degree of water-density. Surface animals would be smothered at a mile of depth; and those brought by dredges from the abysses are usually found to be burst to pieces by the expansion of the air in their cavities and tissues. Thus vertical as well as horizontal limits are set in the sea. Too little is known of the abyssal fauna to say whether its members are world wide or restricted to local areas; but, as the con ditions in the cheep ocean-basins are nearly uni form and undisturbed, it is probable that all the life is widely distributed. See DEEP-SEA Ex PLOIIATION.