Of these we will single out only a few for more special notice.

Many of the Medusidae, or jelly-fish, possess the character of which we are speaking.  In some cases the phosphorescence is spontaneous among them, but in others it is not so; the creature requires to be irritated or stimulated in some way before it will emit the light.  It is spontaneous, for example, in the Pelagia phosphorea, but not in the allied Pelagia noctiluca, a very common form in the Mediterranean.

In both of the jelly-fishes just mentioned the phosphorescence, when displayed at all, is on the surface of the swimming disk, and this is most commonly the case with the whole group.  Sometimes, however, the phosphorescence is specially localized.  In some forms, as in Thaumantius pilosella and other members of the same genus, it is seen in buds at the base of tentacles given off from the margin of the swimming-bell.  In other cases it is situated in certain internal organs, as in the canals which radiate from the centre to the margin of the bell, or in the ovaries.  It is from this latter seat that the phosphorescence proceeds in Oceania pilata, the form which gives out such a light that Ehrenberg compared it to a lamp-globe lighted by a flame.

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.  But we will pass over these cases in order to dwell more particularly on the remarkable tunicate known as Pyrosoma, a name indicative of its phosphorescent property, being derived from two Greek words signifying fire-body.  As shown in the illustration Pyrosoma is not a single creature, but is composed of a whole colony of individuals, each of which is represented by one of the projections on the surface of the tube, closed at one end, which they all combine to form.  The free end on the exterior contains the mouth, while there is another opening in each individual toward the interior of the tube.  Such colonies, which swim about by the alternate contraction and dilatation of the individuals composing them, are pretty common in the Mediterranean, where they may attain the length of perhaps fourteen inches, with a breadth of about three inches.  In the ocean they may reach a much greater size.  Mr. Moseley, in his “Notes of a Naturalist on the Challenger,” mentions a giant specimen which he once caught in the deep-sea trawl, a specimen four feet in length and ten inches in diameter, with “walls of jelly about an inch in thickness.”

[Illustration:  A. PYROSOMA.  B. PONITON. (Magnified.)]

The same naturalist states that the light emitted by this compound form is the most beautiful of all kinds of phosphorescence.  When stimulated by a touch, or shake, or swirl of the water, it “gives out a globe of bluish light, which lasts for several seconds, as the animal drifts past several feet beneath the surface, and then suddenly goes out.”  He adds that on the giant specimen just referred to be wrote his name with his finger as it lay on the deck in a tub at night, and in a few seconds he had the gratification of seeing his name come out in “letters of fire.”