The Phosphorescent Sea

More interesting than the Annelids, however, are the Alcyonarian Actinozoa. The Actinozoa have already been described as formed on the type of the sea-anemone and the coral polyp, that is, they are all animals with a radiate structure, attached to one end, and [pg 237] having their only opening at the other end, which is surrounded by tentacles. In the Alcyonarian forms belonging to this great group these tentacles are always eight in number, and fringed on both sides. Moreover, these forms are almost without exception compound. Like the Pyrosoma, they have a common life belonging to a whole stock or colony, as well as an individual life.

Now, throughout this sub-division of the Actinozoa phosphorescence is a very general phenomenon. Professor Moseley, already quoted as a naturalist accompanying the Challenger expedition, informs us that "all the Alcyonarians dredged by the Challenger in deep water were found to be brilliantly phosphorescent when brought to the surface."

Among these Alcyonarians are the sea-pens mentioned in the quotation above made from Professor Martin Duncan. Each sea-pen is a colony of Alcyonarians, and the name is due to the singular arrangement of the individuals upon the common stem. This stem is supported internally by a coral rod, but its outer part is composed of fleshy matter belonging to the whole colony. The lower portion of it is fixed in the muddy bottom of the sea, but the upper portion is free, and gives off a number of branches, on which the individual polyps are seated. The whole colony thus has the appearance of a highly ornamental pen.

There is one British species, Pennatula phosphorea, which is found in tolerably deep water, and is from two to four inches in length. The specific name again indicates the phosphorescent quality belonging to it. When irritated, it shines brilliantly, and the curious thing is that the phosphorescence travels gradually on from [pg 238] polyp to polyp, starting from the point at which the irritation is applied. If the lower part of the stem is irritated, the phosphorescence passes gradually upwards along each pair of branches in succession; but if the top is irritated the phosphorescence will pass in the same way downwards. When both top and bottom are irritated simultaneously two luminous currents start at once, and, meeting in the middle, usually become extinguished there; but on one occasion Panceri found that the two crossed, and each completed its course independently of the other. Those of our readers who have had opportunities of making or seeing experiments with the sensitive plant (Mimosa pudica) will be reminded of the way in which, when that plant is irritated, the influence travels regularly on from pinnules to pinnules and pinnae to pinnae.

In all the cases mentioned the phenomenon of phosphorescence is exhibited by invertebrate animals; but though rare, it is not an unknown phenomenon even in living vertebrates. In a genus of deep-sea fishes called Stomias, Gunther mentions that a "series of phosphorescent dots run along the lower side of the head, body, and tail." Several other deep-sea fishes, locally phosphorescent, seem to have been dredged up by the French ship Talisman in its exploring cruise off the west coast of Northern Africa in 1883. During the same expedition, a number of deep-sea phosphorescent crustaceans were dredged up, the phosphorescence being in some cases diffused over the whole body, in other cases localized to particular areas. In deep-sea forms the phenomenon is, in fact, so common, as to have given rise to the theory that in the depths of the ocean, where the light [pg 239] of the sun cannot penetrate, the phosphorescence of various organisms diffuse a light which limits the domain of absolute darkness.

So much by way of illustration regarding the phosphorescence exhibited by animals, terrestrial and marine; but it ought to be noticed that there are also a few cases in which the same phenomenon is to be witnessed in plants. These are not so numerous as was at one time supposed, the property having been mistakenly ascribed to some plants not really luminous.

A PHOSPHORESCENT SEA.

In some instances the mistake appears to have been due to a subjective effect produced by brilliantly colored (red or orange) flowers, such as the great Indian cress, the orange lily, the sunflower, and the marigold. The fact that such flowers do give out in the dusk sudden flashes of light has often been stated on the authority of a daughter of Linnæus, subsequently [pg 240] backed by the assertions of various other observers. But most careful observers seem to be agreed that the supposed flashes of light are in reality nothing else than a certain dazzling of the eyes.

In another case, in which a moss, Schistostega osmundacea, has been stated to be phosphorescent, the effect is said to be really due to the refraction and reflection of light by minute crystals scattered over its highly cellular leaves, and not to be produced at all where the darkness is complete.

Among plants, genuine phosphorescence is to be found chiefly in certain fungi, the most remarkable of which is Rhizomorpha subterranea, which is sometimes to be seen ramifying over the walls of dark, damp mines, caverns, or decayed towers, and emitting at numerous points a mild phosphorescent light, which is sometimes bright enough to allow of surrounding objects being distinguished by it. The name of "vegetable glow-worm" has sometimes been applied to this curious growth.

Among other phosphorescent fungi are several species of Agaricus, including the A. olearius of Europe, A. Gardneri of Brazil, and A. lampas of Australia, and besides the members of this genus, Thelaphora cærulea, which is the cause of the phosphorescent light sometimes to be seen on decaying wood—the "touchwood" which many boys have kept in the hope of seeing this light displayed. The milky juice of a South American Euphorbia (E. phosphorea) is stated by Martins to be phosphorescent when gently heated. But phosphorescence is evidently not so interesting and important a phenomenon in the vegetable as it is in the animal kingdom.

The whole phenomenon is one that gives rise to a good many questions which it is not easy to answer, and this is especially true in the case of animal phosphorescence. What is the nature of the light? What are the conditions under which it is manifested? What purpose does it serve in the animal economy?

As to the nature of the light, the principal question is whether it is a direct consequence of the vital activity of the organism in which it is seen, of such a nature that no further explanation can be given of it, any more than we can explain why a muscle is contracted under the influence of a nerve-stimulus; or whether it is due to some chemical process more or less analogous to the burning of a candle.

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