The considerations which we have here developed render it probable that one and the same process operates, alike in the smallest living organisms invisible to the naked eye, in the contests of the serpent-like Gymnoti, in the flashing luminous Infusoria which impart such glorious brilliancy to the phosphorescence of the sea, in the thunder-cloud and in the terrestrial or polar light (the silent magnetic flashes), which, caused by an increased tension of the interior of the earth, are announced, for some hours previously, by the sudden variations of the magnetic needle.[[KL]]
Sometimes one cannot, even with high magnifying powers, discover any animalcules in the luminous water; and yet, wherever a wave breaks in foam against a hard body, and, indeed, wherever water is violently agitated, flashes of light become visible. The cause of this phenomenon depends probably on the decomposing fibres of dead Mollusca, which are diffused in the greatest abundance throughout the water. If this luminous water be filtered through finely woven cloths, the fibres and membranes appear like separate luminous points. When we bathed at Cumana, in the gulf of Cariaco, and walked naked on the solitary beach in the beautiful evening air, parts of our bodies remained luminous from the bright fibres and organic membranes which adhered to the skin, nor did they lose this light for some minutes. If we consider the enormous quantity of Mollusca which animate all tropical seas, we can hardly wonder that sea-water should be luminous, even where no fibres can be visibly separated from it. From the endless subdivision of the masses of dead Dagysæ and Medusæ the whole ocean may, in fact, be regarded as a fluid containing gelatine, and, as such, luminous and of a nauseous taste; unfit for the use of man, but capable of affording nourishment to many species of fish. On rubbing a board with a portion of the Medusa hysocella, the surface thus rubbed recovers its phosphorescence when friction is applied by means of the dry finger. During my voyage to South America I occasionally placed a Medusa on a tin plate, and I then observed that if I struck the plate with another metallic substance the slightest vibrations of the tin were sufficient to cause the animal to emit light. How do the blow and the vibrations here act? Is the temperature momentarily augmented, or are new surfaces presented? or, again, does some gaseous matter such as phosphuretted hydrogen, exude in consequence of this impulse, and burn when it comes in contact with the oxygen of the atmosphere, or with that dissolved in the sea-water, and by which the respiration of the Mollusca is maintained? This light-exciting effect of the blow is most remarkable in a cross or sugar-loaf sea, (mer clapoteuse,) where the waves, clashing from opposite directions, rise in a conical form.
I have seen the ocean, in the tropics, luminous in the most opposite kinds of weather, but most strongly so before a storm, or in a sultry and hazy atmosphere with thick clouds. Heat and cold appear to exercise but little influence on this phenomenon, for, on the Bank of Newfoundland, the phosphorescence is frequently very brilliant in the severest winter. Occasionally, too, the sea will be highly luminous one night, and not at all so on the following, notwithstanding an apparent identity of external conditions. Does the atmosphere favour this development of light? or do all the differences observed during this phenomenon depend on the accidental circumstance of the sea being more or less impregnated, in some parts, with the gelatinous portions of mollusca? Perhaps these phosphorescent social animalcules only rise to the surface under certain conditions of the atmosphere. It has been asked, why our fresh-water swamps which are filled with polyps are not phosphorescent. It would appear that, both in animals and plants, a peculiar mixture of organic particles favours this development of light; thus, for instance, the wood of the willow is more frequently found to be luminous than that of the oak. In England, salt-water has been rendered luminous by mixing herring-brine with it; indeed, it will be easy for any one to convince himself by galvanic experiments, that the luminosity of living animals depends on nervous irritation. I have observed strong phosphorescence emitted from a dying Elater noctilucus, on touching the ganglion of its fore leg with zinc and silver. Medusæ also occasionally emit a stronger light at the moment the galvanic circuit is completed.[[KM]]
[76]. p. 213—“Which inhabits the lungs of the Rattlesnake of the tropics.”
The animal which I formerly named an Echinorhynchus, and to which I even applied the term Porocephalus, appears, on a closer inspection, according to Rudolphi’s better grounded opinion, to belong to the division of Pentastoma.[[KN]] It is found in the abdominal cavity and the wide-celled lungs of a species of Crotalus, which, in Cumana, occasionally infests even the interior of houses, and preys on mice. The Ascaris lumbrici[[KO]] lives beneath the skin of the common earth-worm, and is the smallest of all the species of Ascaris. Leucophra nodulata, Gleichen’s pearl animalcule, has been observed by Otto Friedrich Müller in the interior of the reddish Nais littoralis.[[KP]] It is probable that these microscopic animals are, in their turn, inhabited by others. All are surrounded by air, deficient in oxygen, and copiously charged with hydrogen and carbonic acid. It is extremely doubtful whether any animal could exist in pure nitrogen, although such an opinion did, formerly indeed, seem warranted with reference to Fischer’s Cistidicola farionis, since, according to Fourcroy’s experiments, the swimming-bladder of fish was presumed to contain air wholly devoid of oxygen. But the experiments made by Erman, and confirmed by myself, prove that the swimming-bladder of fresh-water fish never contains pure nitrogen.[[KQ]] In sea fish as much as 0·80 parts of oxygen have been found, while, according to Biot’s views, the purity of the air depends on the depth at which the fishes live.[[KR]]
[77]. p. 214—“The united Lithophytes.”
According to Linnæus and Ellis the calcareous Zoophytes, (among which Madrepores, Meandrinæ, Astrææ, and Pocilloporæ especially produce mural coral-reefs,) are inhabited and invested by animalcules, which were long supposed to be allied to the Nereids belonging to Cuvier’s Annelida (jointed worms). The anatomy of these gelatinous animalcules has been made known by the acute and comprehensive researches of Cavolini, Savigny, and Ehrenberg. We have learned that, in order to understand the whole organism of the (so-called) rock-building animals, we must not consider the scaffolding which remains after their death, namely, the layers of lime formed into delicate lamellæ by a vital function of secretion, as foreign to the soft membranes of the food-receiving animal.
Besides our increased knowledge of the wonderful formation of the living coral-stocks, a more correct view has gradually gained ground respecting the extensive influence which the coral world has exercised on the appearance of low island groups above the level of the sea, on the migration of land-plants, and the successive extension of the domain of the Floras, and, indeed, in some parts of the ocean, on the distribution of the human race and of languages.
As minute social organisms the corals play an important part in the general economy of nature, although they do not, as people began to believe after Capt. Cook’s voyages of discovery, build up islands or enlarge continents from almost unfathomable depths of the ocean. They excite the liveliest interest, whether regarded as physiological objects, and as illustrating the various gradations of animal form, or in connection with the geography of plants, and the geognostic relations of the earth’s crust. According to the comprehensive views of Leopold von Buch, the whole Jura-formation consists of “large elevated coral-banks of the ancient world, surrounding at a certain distance the old mountain chains.”
According to Ehrenberg’s classification,[[KS]] coral-animals, (in English works often incorrectly termed coral-insects,) are separable into the monostomous Anthozoa, which are either free and with the power of detaching themselves, as Animal-corals; or are attached in the manner of plants, as Phyto-corals. To the first order (Zoocorallia) belong the Hydras or Armpolyps of Trembley, the Actiniæ, radiant with the most splendid colours, and the mushroom-corals; and to the second order belong the Madrepores, the Astrææ, and the Ocellinæ. The Polyps of the second order are those which from their cellular, wave-resisting, wall-works are the principal subject of this illustration. The wall-work is composed of the aggregate of the coral-trunks, which, however, do not suddenly lose their combined vitality, like a dead forest tree.