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panied by an evolution of heat. When it is placed in carbonic 
acid gas, the light pales and ceases. On the other hand, the 
photogenic substance, when barely luminous, is rendered so by 
physical contact. Agitation, and the addition of fresh or salt 
water, develop the light, and the same effect is produced by 
electricity and by heat. The light is monochromatic, and has a 
constant place in the spectrum as an azure band from e to r, 
that is to say, in the green, but in the more refrangible part. 
The luminosity of one of the sea-slugs of the Mediterranean 
and Pacific is as remarkable as the creature producing it. 
Living a pelagic life, swimming freely with a fan-like ver- 
tical tail, this little transparent Phyllirhoe bucephala (PI. VI. 
fig. 4) has no shell when in the adult stage, neither has it 
a foot, but its body is compressed and fish-shaped, and it has a 
round and truncated muzzle, behind which are two long flexible 
tentacles. It has no branchiae, and respiration appears to go on 
through the general surface. Now to add to the beauty of this 
translucent creature, light-emission from many distinct round 
spots renders the tissues transparent and luminous. And when 
it is swimming vigorously, the whole surface shines with a diffused 
light. The sexes are combined in this delicate slug, which must 
be a nice morsel for many a fish, and which must find its phos- 
phorescence a fatal gift. There does not appear, however, to 
be any special photogenic substance. The light comes from 
globular cells with an envelope terminating in the outer coat 
of a nerve. The cells are nucleated, and at first sight resemble 
Pacinian bodies without their internal structure. They never- 
theless are terminations of nerves just under the cuticle. (Fig. 4a). 
Some Ophiurans are brilliantly phosphorescent, and it may 
be said from our present knowledge, that those which live at 
considerable depths are more so than the shallow-water forms. 
Their luminosity has no reference to the temperature of the 
surface-water ; and such a species as Ophiacantha spinulosa, 
which has a great bathymetrical range, is intensely brilliant when 
dredged out of very cold water. Sir Wyville Thomson has 
given a very interesting description of the phenomena in “ The 
Cruise of the Porcupine.” He writes : “ Some of these hauls 
were taken late in the evening, and the tangles were spangled 
over with stars of the most brilliant uranian green ; little stars, 
for the phosphorescent light was much more vivid in the younger 
and smaller individuals. The light was not constant nor con- 
tinuous all over the star ; but sometimes it struck out a line of 
fire all round the disc, flashing, or, one might rather say, glow- 
ing up to the centre : then that would fade, and a defined patch, 
a centimetre or so long, break out in the middle of an arm and 
travel slowly out to the point, or the whole five rays would 
light up at the ends and spread the fire inwards. Very young 
