That strange and at times magnificent and imposing phenomenon, the
luminosity of the sea, is certainly due in part to some of the Medusæ.
Members of perhaps all the classes of marine invertebrate animals are
at one time or other engaged in the illumination, and no doubt the most
wide-spread production of spontaneous light, and the most effective, is
due to creatures which are individually unrecognisable by the eye. When
the ship ploughing through the tropical sea, turns luminous furrows
on each side of her prow, and leaves a long wake of curdling light
astern, or when the steamer dashes the water of our own estuaries into
cascades of fire and showers of coruscating sparks, it is doubtless
to the microscopic _Infusoria_, _Annelida_, and _Entomostraca_, that
we are mainly indebted for the charming spectacle. Still, many of the
Medusæ are conspicuously luminous under certain conditions, generally
displaying the phenomenon at the moment of irritation; the light being
evolved, not apparently by any proper organs, but either by the whole
of the marginal ring, or by the (often coloured) swellings that are
seated at the base of the tentacles.
The following appearances proved the luminosity of this beautiful
_Æquorea_, on being subjected to experiments in the dark. When with
my finger-nail I tapped the glass jar in which two specimens were
floating at the surface, instantly each became brilliantly visible as a
narrow ring of light, the whole marginal canal becoming luminous. On my
touching them with the end of a stick, the light became more vivid, and
round spots appeared here and there in the ring, of intense lustre and
of a greenish-blue tint. These were, I doubt not, the tentacle-bulbs;
and any one of them would be excited to this intensity by my touching
that part of the margin with the stick. The luminosity of the ring was
not so evanescent as in some species, lasting several seconds, and
continuing to be renewed as often as I molested the animal. The two
circles of light, two inches or more in diameter, were very beautiful
as they moved freely in the water, sinking or rising according as they
were touched, now seen in full rotundity, now shrinking to an oval, or
to a line, as either turned sidewise to the eye, and reminded me of
the _coronæ_ of glory in the pictures of the Italian school, round the
heads of saints.