unicates, 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.

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 t