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 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 of the sun cannot penetrate, the phosphorescence of various organisms diffuse a light which limits the domain of absolute darkness.