Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/295

277 ANIMAL.] face-dwellers the broad expanse of warm water between tho tropics, with its hosts of specially adapted organisms, forms an absolute barrier. In like manner the inhabitants of the tropical shallow waters are limited, and it is only by tem porary subsidences of land or elevations of the sea-bottom to near the surface, allowing of a passage east or west, that they can migrate into remote areas. We -have good reason to believe, however, that subsidences have often occurred between North and South America, allowing of a free inter change of aquatic animals between the Atlantic and Pacific oceans ; while in Eocene times a strait is supposed to have connected the Atlantic and Indian oceans, and more recently the Ked Sea and Mediterranean have almost cer tainly been united. We cannot, therefore, expect to find any such strongly-marked zoological regions among aquatic as among terrestrial animals, and the facts at our command entirely confirm this view. For many groups the warm and the cold, or the northern, tropical, and southern seas, are the only well-marked divisions ; while for others the North Atlantic, the North Pacific, and the Indian Ocean form additional regions of a more or less defined character. For special studies of the more highly-organized marine groups as the Mollusca or Crustacea a host of provinces and sub-provinces have been formed, each important sea or coast presenting some peculiar features ; but as these divi sions mostly depend on specific rather than generic distri bution, they need hardly be noticed here. Owing to the absence of any well-marked regions, and the general imperfection of our knowledge of the distribu tion of marine animals, we must follow a different plan in our sketch of this subject from that adopted for the terrestrial fauna. We propose, then, to notice successively the more important classes of marine animals, and to state briefly what general facts are established as to their distri bution. Foraminifera. These exceedingly low organisms are im portant, because their shells or tests are found extensively in various geological formations, and often form a considerable part of the constituents of rocks. They are found in all seas, and the species have often an enormous range. The surface swimmers are almost universally distributed, while the bottom-livers as the writer is informed by Mr H. B. Brady appear to be distributed according to depth and latitude rather than to follow any circumscribed areas. This gentleman is now engaged in working out the &quot; Challenger &quot; collections, and already sees reason to think that there may be found some differences between Atlantic and Pacific, and also between North Atlantic and South Atlantic forms. These organisms have been found living in the surface waters and down to a depth of 2000 fathoms, but only their dead remains are brought up from the floor of the deep ocean. Spongida. Sponges form another extensive group, often preserved as fossils, of which our knowledge of the recent forms is rapidly increasing. There seems, however, to be little geographical, limitation of groups. The fresh water Spongillidae are found in all regions. The horny and calcareous sponges are widely distributed, but are mitch more abundant in warm and tropical seas. The beautiful siliceous sponges of which the glass-rope (Hyalonema) and Venus s flower-basket (Euplectella) are conspicuous examples are found scattered throughout all warm seas, and have recently been found in abundance in many of the great ocean depths, in the Atlantic at 2C50 fathoms, and in the Pacific at 3000 fathoms, so that they probably exist wherever the nature of the bottom is favourable. (Sir Wyville Thomson, Depths of the Sea ; &quot;Keports from the Challenger, &quot; Proc. Pay. Soc., vol. xxiv. ; Bowerbank s &quot; Papers on Sponges &quot; in Proceedings of Zoological Society, 1869-1874.) Actinozoa. The coral-bearing groups of these aniimils are the more important, because of the abundance of fossil forms of every geological age. By far the greater number of these are found exclusively in tropical seas. Many of the Alcyonaria are temperate and even Arctic, while among the Zoantharia the Cari/opkyllidce alone are well repre sented beyond the tropics. The distribution of corals is in great part determined by the physical conditions of the sea- bottom. An influx of fresh water or of mud brought down by rivers is fatal to them, and volcanic deposits seem to be almost equally prejudicial. A high temperature is also necessary for most of the groups. Coral reefs are therefore restricted to certain seas and coasts within or near the tropics. They abound in and near the West Indies, on the east coast of Africa, in the Indian Ocean, in the Malay and Pacific archipelagoes, and on the coast of Australia; while they are absent from the whole of the west coasts of South America and of Africa, from the Indian peninsula, and from much of the east coast of South America. The coral-reefs of the Bermudas, in 33 N. lat, are the farthest from the equator ; in the Red Sea they reach 30 N., in the Pacific 27 N.. while they nowhere extend to more than 29 S. of the equator. Besides the corals actually forming the reefs, the same localities abound in what are termed deep-sea corals, and thus the general distribution of the group is determined by similar conditions. The coral regions are therefore somewhat peculiar, and differ con siderably from those which best exhibit the distribution of other marine animals. The regions adopted by Mr Dana are three, the first comprising the Red Sea and Indian Ocean ; the second, the whole of the Pacific islands and the adjacent coasts of Australia ; and the third the West Indies. This last region is the most isolated in position, and it is not surprising that it should contain the largest proportion of peculiar forms. The corals of the Central Pacific are also very peculiar, as are those of the Red and Indian seas. Considering the great similarity of the molluscan fauna of the Pacific and Indian oceans, it is remarkable that the corals should be so different as Mr Dana s tables show them to be. Although some corals exist at great depths in the ocean, they diminish rapidly when we pass the moderate depth of 200 fathoms. The &quot; Challenger &quot; expedition obtained 27 genera at a greater depth than 250 fathoms, but only 3 of these extended below 1500 fathoms. Count Pourtales on the American coast found them at 400 fathoms ; the &quot; Porcupine &quot; expedition first found them at a greater depth than 1000 fathoms in the North Atlantic ; while in the Pacific a single species has been found at the enormous depth of 2900 fathoms. The following genera were obtained by the &quot; Challenger &quot; dredgings at a greater depth than 1000 fathoms : Caryophyllia, Deltocyathus, Ccratotrochus, Flabelhim, Amphehelia, Cryptohelia, and Fitngia. Some of these were of brilliant colours pink, madder-red, white, and emerald green. A large proportion of fossil genera of corals survive in the deep seas, no less than seven genera, before only known in the fossil state, having been added by the explorations of the &quot; Challenger ; &quot; seven of these are found at or below 1000 fathoms. It is curious, how ever, that the deepest by far of all corals, Fungia, is not known to be fossil, nor is any member of the family Stylasteridce, six genera of which are deep-sea corals ; so that too much importance must not be attached to the fact of genera hitherto only known as fossils having been found living at great depths in the ocean. (Dana s &quot; Zoophytes &quot; in U.S. Exploring Expedition, vol. vii. ; &quot; Report on Corals Dredged by the Challenger, &quot; by H. N. Moseley, Proc. Roy. Soc., vol. xxiv. p. 544.) Polyzoa. The coral-like Molluscoida forming the exten sive group of marine polyzoa, have been carefully studied,