FACTS AND THOUGHTS ABOUT LIGHT-EMITTING ANIMALS. 229 
mouth and very minute and fibrillar near the outside, is a 
denser sarcode, and there are granules on the fibrils. These 
fibrillar meshes are enveloped in a minutely-granular sarcode, 
and they reach to just under the surface of the animal, ending 
in a clear protoplasmic layer, which underlies the equally clear 
and transparent cuticle or cell-wall. Near the vacuolated part 
is a small nucleus, and it is evidently in relation with the 
fibrillar radiations ; and there are occasionally nucleated cell- 
like bodies in the peripheral layer of protoplasm. As these are 
probably spores, it is not necessary to consider them. Now 
there is movement in the striated tentacle and in the long fila- 
mentous cilium, and there is amoeboid streaming of granules 
on the radiating fibrils, but no change of general shape constantly 
occurs. The animal respires by its outer cell-wall, grows in 
size by additions to the finer granular parts, and the principal 
seat of this activity must be just beneath the cell-wall. 
Now, on watching Noctiluca in captivity, one is struck by 
the very vivid light which it emits. It is a sudden flash, lasting 
but for a short time, and is repeated over and over again, some 
intermission being apparently inevitable and necessary. The 
light is greenish, and is produced not especially near the seat 
of tentacular and fibrillar movement, but just under the cell- 
wall. It arises from scores of minute, independent points, 
which scintillate and illuminate the rest : in fact, where respira- 
tion and assimilation are at their greatest, where the vital energy 
is in full action. Anything which increases this activity pro- 
duces increase of the light, and the converse is true. Pure, 
highly aerated sea-water, changed over and over again, adds to 
its brilliancy and persistence. Oxygen forced into the water 
produces more light, and the stimulation of fresh (non-saline) 
water at first does the same, but sooner or later it is destructive 
to the animal. Physical stimulation evidently acts on the light, 
and produces it for a time, and a constant illumination pre- 
cedes death, when light ceases. The light diminishes in vacuo , 
and under the influence of carbonic acid gas. 
Quatrefages experimented upon Noctiluca , and added alcohol ; 
and this produced a definite continuous luminous ring, and then a 
general peripheral illumination, which lasted for a while and 
until death. Finally, it is well known that nets which have 
been dragged out of a sea crowded wdth Noctiluca , retain light- 
emitting powers until the meshes become dry. The sun-light 
has nothing to do with the luminosity of Noctiluca. 
The Spongidae have not as yet been recorded as having lu- 
minous kinds, but the Hydrozoa teem with them ; and more- 
over the gift, so common in the planoblasts, is found in some 
species in the stationary and vegetative animal. Probably nearly 
all Medusae are luminous, and notably the larger Aurelice 
