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The property of emitting a phosphorescent light, sometimes spontaneously and sometimes on being stimulated, is likewise exemplified in the Ctenophora, a group resembling the Medusidge in the jelly-like character of their bodies, but more closely allied in structure to the Actinozoa.
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.
This lovely fairy-like globe, with its long floating tentacles and rainbow tints, was for a long time classed with the jelly-fish; but it really is most nearly related to the sea-anemones, as it has a true central cavity which acts as a stomach, and many other points in common with the 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 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.
If we come now to consider the more partial phosphorescence of the sea, we find that it is due to animals belonging to almost every group of marine forms to Echinoderms, or creatures of the sea-urchin and star-fish type, to Annelid worm, to Medusidæ, or jelly-fish, as they are popularly called, including the "great domes" and the "silvery disks" of the passage above quoted from Professor Martin Duncan, to Tunicates, among which is the Pyrosoma, to Mollusks, Crustaceans, and in very many cases to Actinozoa, or forms belonging to the type of the sea anemone and the coral polyp.
He then found, as he did in the great majority of cases which he studied, that the phosphorescence was confined to the motor muscles, and was manifested solely when these were in the act of contracting, manifested, too, not in continuous lines along the course of the muscles, but in rows of brilliant points. More interesting than the Annelids, however, are the Alcyonarian Actinozoa.
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