hey 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."

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,