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exceptional brilliancy when the animal is withdrawn from the water and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.

Now, throughout this sub-division of the Actinozoa phosphorescence is
a very general phenomenon. Professor Moseley, already quoted as a
naturalist accompanying the Challenger expedition, informs us that
"all the Alcyonarians dredged by the Challenger in deep water were
found to be brilliantly phosphorescent when brought to the surface."

Among these Alcyonarians are the sea-pens mentioned in the quotation
above made from Professor Martin Duncan. Each sea-pen is a colony of
Alcyonarians, and the name is due to the singular arrangement of the
individuals upon the common stem. This stem is supported internally by
a coral rod, but its outer part is composed of fleshy