huge Astræa domes, the Meandrinas, Porites, the leafy clusters of the Merulinæ, numerous Madrepores:-indeed nearly all the Pacific corals described in the Report on Zoophytes, exclusive of those from the Tahitian and Hawaiian Islands, were obtained from the inner reefs of the Feejees.* It is therefore an assertion wide from the fact, that only smaller corals grow in the lagoons and channels, though true of lagoons and channels of small size, or of such parts of the larger channels as immediately adjoin the mouths of fresh water streams. There are undoubtedly species especially fitted for the open ocean; but as peculiar conveniences are required for the collection of zoophytes outside of the line of breakers, we have not the facts necessary for an exact list of such species. From the very abundant masses of Astræas, Meandrinas, Porites, and Madreporas thrown up by the waves on the exposed reefs, it was evident that these genera were well represented in the outer seas. In the Paumotus, the single individuals of Porites lying upon the shores were at times six or eight feet in diameter. Around the Duke of York's Island the bottom was observed to be covered with small branching and foliaceous Madrepores (Manopore), as delicate as any of the species in more protected waters. Species of the same genera grow in the face of the breakers, and some are identical with those that occur also in deeper waters. Numerous Astræas, Meandrinas, and Madreporas grow at the outer edge of the reefs, where the waves come tumbling in with their full force. There are also many Milleporas, and some Porites and Pocilloporas in the same places. But the weaker Manoporas, excepting incrusting species, are found in stiller waters, either deep or shallow. The Nullipores, properly calcareous vegetation, flourish best along the line of breakers, and form thick accumulations upon the reef.† * The author's observations on the species of corals were not commenced till reaching the Feejees, where we were among the inner reefs. Previous to that time, this department in zoology was in the hands of Mr J. P. Couthouy. + Porites and Milleporæ, according to Mr Darwin, prevail on the surf-reef of Keeling's Island. Chamisso states that the large Astræas live and grow in the breakers. Again, the same genera occur in the shallow waters of the reef inside of the breakers. Astræas, Meandrinas, and Pocilloporas are not uncommon, though requiring pure waters. There are also Madreporas, some growing even in impure waters. One species was the only coral observed in the lagoon of Honden Island (Paumotus), all others having disappeared, owing to its imperfect connection with the sea. Upon the reefs inclosing the harbour of Rewa (Viti Lebu), where a large river, 300 yards wide, empties, which during freshets enables vessels at anchor two and a-half miles off its mouth to dip up fresh water alongside, there is a single porous species of Madrepora (M. cribripora) growing here and there in patches over a surface of dead coral rock or sand. In similar places about other regions, species of Porites are most common. In many instances, the living Porites are seen standing six inches above low tide, where they were exposed to sunshine and to rains; and associated with them in such exposed situations, there were usually great numbers of Alcyonia and Xenia. Porites also occur in the impure waters adjoining the shores; and the massive species in such places, commonly spread out into flat disks, the top dying from the deposition of sediment upon it. The exposure of six inches above low tide, where the tide is six feet, as in the Feejees, is of much shorter duration than in the Paumotus, where the tide is less than half this amount; and consequently the height of growing coral, as compared with low-tide level, varies with the height of the tides. The powers of endurance in some coral zoophytes cannot surprise us, for it is well known that these animals are often very tenacious of life. The hardier species belong mostly to the genera Porites and Pocillopora, besides the family Alcyonidæ. The small lagoons, when shut out from the influx of the sea, are often rendered too salt for growing zoophytes, in consequence of evaporation,—a condition of the lagoon of Enderby's Island. Coral zoophytes sometimes suffer injury from being near large fleshy Alcyonia, whose crowded, drooping branches lying over against them, destroy the polyps, and mar the growing mass. But Serpulas and certain species of bar nacles, constituting the genus Criseis, fix themselves upon the living Astræa, Millepora, and other corals, and finally become imbedded by the increase of the zoophyte, without producing any defacement of the surface, or affecting its growth. Many of the Serpulas grow with the same rapidity as the zoophyte, and finally produce a long tube, which penetrates deep within the coral mass; and, when alive, they expand a large and brilliant circle or spiral of delicate rays, making a gorgeous display among the coral polyps. Instinct seems to guide these animals in selecting those corals which correspond with themselves in rate of growth; and there is in general a resemblance between the markings of a Criseis and the character of the radiations of the Astrea it inhabits. The effects of sediment on growing zoophytes are strongly marked, and may be often perceived when a mingling of fresh water alone produces little influence. We have mentioned that the Porites are reduced to flattened masses by the lodgment of sediment. The same takes place with the hemispheres of Astræa; and it is not uncommon that in this way large areas at top are deprived of life. The other portions still live unaffected by the injury thus sustained. Even the Fungiæ, which are broad simple species, are occasionally destroyed over a part of the disk through the same cause, and yet the rest remains alive. Wherever streams or currents are moving or transporting sediment, there no corals grow; and for the same reason we find no living zoophytes upon sandy or muddy shores. The influence of temperature on the development of animal life, and the distribution of species, is well known. But in no department is it more strikingly displayed than in that of zoophytes. In a former report we have considered the general influence of temperature on the several divisions of this order of animals. The remarks which follow are consequently confined to the reef-forming species. We reserve for still another page the influence of this cause on the distribution of reefs, since we are occupied here with zoophytes as animal species, and not with reefs,-a result from the growth of corals. The temperature of the ocean in which reef-corals grow is evidently the temperature congenial to them. From a general survey of facts, it appears that these species are not met with where the winter temperature remains much time below 66° Fahr., though a temporary reduction to 64°, or perhaps lower (as the Bermudas), may sometimes occur. Where the temperature is above this, even in the hottest parts of the torrid zone, coral zoophytes thrive well. An isothermal line, crossing the ocean where this winter temperature of the sea is experienced, one north of the equator, and another south, bending in its course by divergence or convergence, wherever the marine currents change its position, will include all the growing reefs of the world; and the area of waters may be properly called the coral-reef seas. This limiting temperature is found near latitude 28°. Under the equator in the Pacific, the waters where warmest have the temperature 85° Fahr., and in the Atlantic 83° Fahr.; 66° to 85° is therefore not too great a range of temperature for the various reef-forming corals. Particular species, however, have similar limits; but these limits have not yet been accurately ascertained.* The Porites and Pocillopora predominate at Oahu (Sandwich Islands), and there are but few of the Astræidæ,—a fact which appears to be explained on the ground that the reefs of that island are not far from the cold limits of the coral seas; and it is interesting to observe that these same corals are the hardiest under exposure to impure waters. The warmest parts of the ocean are favourable to the growth of Astræas, Meandrinas, and the allied species; and at the same time, these regions abound in Porites and Pocillopora, although the proportion of these corals is smaller than at Oahu. The genera of reef-forming corals which occur out of the coral-reef seas, belong almost exclusively to the Caryophyllia family, and especially to the genera Dendrophyllia, Caryo * The first application of the well-established principle that temperature influences the growth and distribution of corals is claimed by Mr J. P. Couthouy equally with myself. Any attempt, however, to determine a limiting temperature he declaims, and in this particular, as well as the conclusions arrived at, our views are very different. The facts and inferences stated in this place, and on a following page, are deduced throughout from my own study and investigation. phyllia, Astroides,* Oculina, and Cyathina, some species of which exist in the Norwegian seas. The Gorgonidæ, Alcyonidæ, Hydroidea, and Actinidæ, extend from the equator nearly to the frigid zone. The Bryozoa have an equally wide range.† The liability of the lagoons, when contracted in size, to become highly heated by the sun, is probably one cause leading to the depopulation of these internal waters. The temperature becomes raised, as in a puddle of standing water elsewhere, and is quite unfitted, therefore, for species accustomed only to the ordinary tropical temperature of the ocean. Light and pressure, and probably the amount of air in seawater, influence the growth of corals so far as to fix limits to their distribution in depth. It is a little remarkable that those families which have a wide geographical range have also a great range in depth; for Carophylliæ, Dendrophylliæ, Oculinæ, Gorgonidæ, and. Hydroidea, are found even at depths of one or two hundred fathoms; while Madrepores and Astræas, and all the ordinary reef-forming species scarcely exceed a depth of twenty fathoms. Temperature has little or no influence in determining this range, although it has been so asserted; 66° is not met with under the equator short of 75 or 100 fathoms. The following table gives approximate results for the winter months from observations on this point by different navigators in the Pacific. It is well known that these averages are varied much in particular regions by currents. *The corals of the Astroides closely resemble those of the Astræa, and have been referred to the latter group by many authors. A related species is found on the coast of this country as high up as latitude 42°. An Astræa has been reported from Sydney, New South Wales, which, if a true Astræa (it has not been described or figured), gives this genus a wider limit than the coral-reef seas. See farther, Report on Zoophytes, p. 102. |