Among mollusks, the best known instance of phosphorescence is in the rock-boring Pholas, the luminosity of which after death is mentioned by Pliny.  But it is not merely after death that Pholas becomes luminous—­a phenomenon perfectly familiar even in the case of many fish, especially the herring and mackerel.  It was long before the luminosity of the living animal was known, but this is now a well-ascertained fact; and Panceri, an Italian naturalist, recently dead, has been able to discover in this, as in several other marine phosphorescent forms, the precise seat of the light-giving bodies, which he has dissected out again and again for the sake of making experiments in connection with this subject.

A more beautiful example of a phosphorescent mollusk is presented by a sea-slug called Phyllirhoe bucephala.  This is a creature of from one and a half to two inches in length, without a shell in the adult stage, and without even gills.  It breathes only by the general surface of the body.  It is common enough in the Mediterranean, but is not easy to see, as it is almost perfectly transparent, so that it cannot be distinguished without difficulty, by day at least, from the medium in which it swims.  By night, however, it is more easily discerned, in consequence of its property of emitting light.  When disturbed or stimulated in any way, it exhibits a number of luminous spots of different sizes irregularly distributed all over it, but most thickly aggregated on the upper and under parts.  These phosphorescent spots, it is found, are not on the surface, but for the most part represent so many large cells which form the terminations of nerves, and are situated underneath the transparent cuticle.  The spots shine with exceptional brilliancy when the animal is withdrawn from the water and stimulated by a drop of ammonia.

Among the Annelid worms a species of Nereis, or sea-centipedes, has earned by its phosphorescent property the specific name of noctiluca (night-shining), and the same property is very beautifully shown in Polynoe, a near ally of the familiar sea-mouse.  M. de Quatrefages speaks with enthusiasm of the beauty of the spectacle presented by this latter form when examined under a microscope magnifying to the extent of a hundred diameters.  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.  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.  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.