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ANIMAL LUMINOUSNESS. 
In elater ignitus the masses of luminous sub- 
stance are extremely irregular in their figure ; 
they are situated close to the posterior angles 
of the corselet, and are more loose in their 
structure than the same parts in elator noctilucus. 
The luminous proboscis or snout of the 
Julgora is hollow, and has a free communica- 
tion with the external air by a narrow slit 
situated near the base of the organ. Its cavity 
is lined with a fine membrane, between which 
and the outer translucent corneous crust, there 
is interposed a soft tissue of a pale reddish 
colour, arranged in lines longitudinally, which 
is supposed to be the seat of luminousness in 
this insect. 
In several instances it has been found that 
the light-giving substance has continued to 
shine for a considerable time after being re- 
moved from the body of the insect. In such 
cases it has been observed that there appeared 
to he no diminution either in the weight or the 
bulk of the luminous organs, excepting what 
was obviously produced by evaporation of the 
fluids. 
VI. Geographical distribution of luminous 
animals.—In almost all seas, in every latitude 
from 60° S. to 80° N., have light-giving ani- 
mals been seen; but they are more abundant, 
and shine with greater brilliancy, in the tropical 
than in the colder climates. In general, it is 
observed by voyagers, the luminous mollusca 
and acalephe occur in greatest numbers not far 
from land ; and that they are particularly plen- 
tiful in the seas surrounding groups of small 
islands within the tropics. 
The luminous insects are met with chiefly 
in the warmer climates of the temperate zones 
and within the tropics. We are not aware of 
any having been met with beyond the latitude 
of 58°. 
VIL. Theories of animal luminousness.— 
Very numerous have been the theories formed 
by philosophers with regard to the nature of 
the luminous matter which produces the phe- 
nomena now under review. From the facts 
Stated above, it appears that we are not yet in 
a position to determine with certainty whether 
or not animal luminousness has its source in 
the operations of any agent already known. 
At least it appears to us that facts enough have 
been accumulated to set aside the assumption 
of most of the theories hitherto promulgated. 
The following are some of these. 
1. That light is imbibed from the sun’s rays 
by luminous animals and given out in the 
dark. (Beccaria, Mayer, &c.) 
2. That the light is owing to a kind of com- 
bustion maintained by the oxygen of the air. 
(Spallanzani.) 
3. That light is swallowed with the food, 
oy disengaged by peculiar organs. (Brugna- 
telh. 
4. That the light-giving matter is composed 
of phosphorus and albumen; and that the 
variations in the intensity of the light depend 
on the more or less complete coagulation of the 
albumen, by some internal means placed under 
the control of the animal’s will. (Macaire.) 
203 
5. That a fluid containing phosphorus is 
secreted by the luminous organs, and shines 
on its being exposed to the oxygen of the air 
introduced by respiration. (Darwin, H. Davy, 
Heinrich, Treviranus, and Burmeister.) 
6. That the luminous organs concentrate 
and modify the nervous influence so as to form 
it into light; so that, according to this theory, 
animal luminousness is an effect solely of vital 
power. (Macartney and Todd.) 
7. Tiedemann thus expresses his opinion: 
“ Animal luminousness would seem to depend 
on a matter, the product of the changes of 
composition accompanying life, and, to all ap- 
earance, secreted from the mass of humours 
y particular organs. This liquid probably 
contains phosphorus or an analogous combus- 
tible substance, which combines with the oxy- 
gen of the air or of aérated water at a medium 
temperature, and thus produces the disengage- 
ment of light. The preparation and secretion 
of this substance are acts of life, which change, 
augment, or decrease by the influence of ex- 
ternal stimulants, whose action on the animals 
modifies their manifestations of life. But the 
phosphorescence itself depends on the com- 
position of the secreted matter and cannot be 
regarded as a vital act; because, on certain 
occasions, it continues for whole days and even 
after the death of the animal.” * 
This opinion seems to coincide pretty nearly 
with that of Darwin, Heinrich, and others, 
stated above (5); and we must admit that it ap- 
pears toharmonize the facts better than any of the 
other theories that have been propounded. But, 
while it seems satisfactorily to assimilate many 
of the phenomena to others more familiar to us, 
and more within the reach of our investigations, 
and thus appears to furnish future inquirers 
with a key to the elucidation of what yet re- 
mains obscure, it is obvious that it leaves some 
of the phenomena unexplained, and that se- 
veral of these seem to be quite irreconcilable 
with the theory of the phosphorescence being 
essentially dependent on the composition of 
secreted fluids. In some of the extremely 
delicate acalephe, for instance, from which the 
brightest radiance is so frequently emitted in 
momentary fitful flashes, it is difficult to con- 
ceive why, if the light mainly depended on 
the nature of some matter poured out by cer- 
tain organs at the instant of the flash, the light 
should not continue for at least a few seconds. 
The circumstance of its doing so in some in- 
stances, and even mixing gradually with the 
surrounding sea-water, certainly proves that 
there is such a fluid as this theory supposes in 
certain animals; but does not remove the dif- 
ficulty presented by the facts we have al- 
luded to. 
We feel ourselves constrained by these and 
other such facts to believe (with Macartney 
and Todd) that in many, perhaps in all, lumi- 
nous animals, both terrestrial and marine, the 
light emitted is the consequence of an evolu- 
* Comparative Physiology, translated by Drs. 
Gully and Lane, i. 270. 
