Sometimes one cannot even with high magnifying powers discern any animalcules in the luminous water; and yet, whenever the wave strikes and breaks in foam against a hard body, a light is seen to flash. In such case the cause of the phenomenon probably consists in the decaying animal fibres, which are disseminated in immense abundance throughout the body of water. If this luminous water is filtered through fine and closely woven cloths, these little fibres and membranes are separated in the shape of shining points. When we bathed at Cumana in the waters of the Gulf of Cariaco, and afterwards lingered awhile on the solitary beach in the mild evening air without our clothes, parts of our bodies continued luminous from the shining organic particles which had adhered to the skin, and the light only became extinct at the end of some minutes. Considering the enormous quantity of animal life in all tropical seas, it is, perhaps, not surprising that the sea water should be luminous, even where no visible organic particles can be detached from it. From the almost infinite subdivision of the masses of dead Dagysæ and Medusæ, the sea may perhaps be looked on as a gelatinous fluid, which as such is luminous, distasteful to, and undrinkable by man, and capable of affording nourishment to many fish. If one rubs a board with part of a Medusa hysocella, the part so rubbed regains its luminosity on friction with a dry finger. On my passage to South America I sometimes placed a Medusa on a tin plate. When I struck another metallic substance against the plate, the slightest vibrations of the tin were sufficient to cause the light. What is the manner in which in this case the blow and the vibrations act? Is the temperature momentarily augmented? Are new surfaces exposed? or does the blow press out a fluid, such as phosphuretted hydrogen, which may burn on coming into contact with the oxygen of the atmosphere or of the air held in solution by the sea-water. This light-exciting influence of a shock or blow is particularly remarkable in a “cross sea,” i. e. when waves coming from opposite directions meet and clash.
I have seen the sea within the tropics appear luminous in the most different states of weather; but the light was most brilliant when a storm was near, or with a sultry atmosphere and a vaporous thickly-clouded sky. Heat and cold appear to have little influence on the phenomenon, for on the Banks of Newfoundland the phosphorescence is often very bright during the coldest winter weather. Sometimes under apparently similar external circumstances the sea will be highly luminous one night and not at all so the following night. Does the atmosphere influence the disengagement of light, or do all these differences depend on the accident of the observer sailing through a part of the sea more or less abundantly impregnated with gelatinous animal substances? Perhaps it is only in certain states of the atmosphere that the light-evolving animalculæ come in large numbers to the surface of the sea. It has been asked why the fresh water of our marshes, which is filled with polypi, is never seen to become luminous. Both in animals and plants, a particular mixture of organic particles appears to be required in order to favour the production of light. Willow-wood is oftener found to be luminous than oak-wood. In England experiments have succeeded in making saltwater shine by pouring into it the liquor from pickled herrings. It is easy to shew by galvanic experiments that in living animals the evolution of light depends on an irritation of the nerves. I have seen an Elater noctilucus which was dying emit strong flashes of light when I touched the ganglion of his fore leg with zinc and silver. Medusæ sometimes shew increased brightness at the moment of completing the galvanic circuit. (Humboldt, Relat. Hist. T. i. p. 79 and 533.)
Respecting the wonderful development of mass and power of increase in Infusoria, see Ehrenberg, Infus. S. xiii. 291 and 512. He observes that “the galaxy of the minutest organisms passes through the genera of Vibrio and Bacterium and that of Monas, (in the latter they are often only 1⁄3000 of a line,)” S. xix. and 244.
[6] p. 7.—“Which inhabits the large pulmonary cells of the rattle-snake of the tropics.”
This animal, which I formerly called an Echinorhynchus or even a Porocephalus, appears on closer investigation, and according to the better founded judgment of Rudolphi, to belong to the division of the Pentastomes. (Rudolphi, Entozoorum Synopsis, p. 124 and 434.) It inhabits the ventral cavities and wide-celled lungs of a species of Crotalus which lives in Cumana, sometimes in the interior of houses, where it pursues the mice. Ascaris lumbrici (Gözen’s Eingeweidewürmer, Tab. iv. Fig. 10,) lives under the skin of the common earthworm, 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. (Müller, Zoologia danica, Fasc. II. Tab. lxxx. a—e.) Probably these microscopic animals are again inhabited by others. All are surrounded by air poor in oxygen and variously mixed with hydrogen and carbonic acid. Whether any animal can live in pure nitrogen is very doubtful. It might formerly have been believed to be the case with Fischer’s Cistidicola farionis, because according to Fourcroy’s experiments the swimming bladders of fish appeared to contain an air entirely deprived of oxygen. Erman’s experience and my own shew, however, that fresh-water fishes never contain pure nitrogen in their swimming bladders. (Humboldt et Provençal, sur la respiration des Poissons, in the Recueil d’Observ. de Zoologie, Vol. ii. p. 194-216.) In sea-fish as much as 0·80 of oxygen has been found, and according to Biot the purity of the air would appear to depend on the depth at which the fish live. (Mémoires de Physique et de Chimie de la Societé d’Arcueil, T. i. 1807, p. 252-281.)
[7] p. 8.—“The collected labours of united Lithophytes.”
Following Linnæus and Ellis, the calcareous zoophytes,—among which Madrepores, Meandrinæ, Astreæ, and Pocilloporæ, especially, produce wall-like coral-reefs,—are inhabited by living creatures which were long believed to be allied to the Nereids belonging to Cuvier’s Annelidæ. The anatomy of these gelatinous little creatures has been elucidated by the ingenious and extensive researches of Cavolini, Savigny, and Ehrenberg. We have learnt that in order to understand the entire organization of what are called the rock-building coral animals, the scaffolding which survives them, i. e., the layers of lime, which in the form of thin delicate plates or lamellæ are elaborated by vital functions, must not be regarded as something extraneous to the soft membranes of the food-receiving animal.
Besides the more extended knowledge of the wonderful formation of the animated coral stocks, there have been gradually established more accurate views respecting the influence exercised by corals on other departments of Nature,—on the elevation of groups of low islands above the level of the sea,—on the migrations of land-plants and the successive extension of the domains of particular Floras,—and, lastly, in some parts of the ocean, on the diffusion of races of men, and the spread of particular languages.
As minute organic creatures living in society, corals do indeed perform an important part in the general economy of Nature, although they do not, as was begun to be believed at the time of Cook’s voyages, enlarge continents and build up islands from fathomless depths of the ocean. They excite the liveliest interest, whether considered as subjects of physiology and of the study of the gradation of animal forms, or whether they are regarded in reference to their influence on the geography of plants and on the geological relations of the crust of the Earth. According to the great views of Leopold von Buch, the whole formation of the Jura consists of “large raised coral-banks of the ancient world surrounding the ancient mountain chains at a certain distance.”
In Ehrenberg’s Classification, (Abhandlungen der Akad. der Wiss. zu Berlin aus dem, J. 1882, S. 393-432) Coral-animals, (often improperly called, in English works, Coral-insects) are divided into two great classes: the single-mouthed Anthozoa, which are either free or capable of detaching themselves, being the animal-corals, Zoocorallia; and those in which the attachment is permanent and plant-like, being the Phyto-corals. To the first order, the Zoocorallia, belong the Hydras or Arm-polypi of Trembley, the Actiniæ decked with beautiful colours, and the mushroom-corals; to the second order or Phyto-corals belong the Madrepores, the Astræids, and the Ocellinæ. The Polypi of the second order are those which, by the cellular wave-defying ramparts which they construct, are the principal subject of the present note. These ramparts consist of an aggregate of coral trunks, which, however, do not instantly lose their common vitality as does a forest tree when cut down.