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[ Sef. 17, 1885 
One of the most striking instances of phosphorescence in 
living fishes is that of the luminous shark (Sgealus fulgens) 
found by Dr. Bennett. This is a small dark-coloured shark, 
which was captured on two or three occasions at the surface of 
the sea. It emitted without irritation a vivid greenish luminosity 
as it swam about at night, and it shone for some hours after death. 
The phosphorescence appears to be due to a peculiar secretion 
of the skin. The eyes of the shark were more prominent than 
usual insuch forms. (The Danish naturalist Reinwardt describes 
a phosphorescent fish (Hemiramphus lucens) from the Moluccas. 
Fide Giglioli, Of. cit. p. 503.) Little is known with regard to 
the luminosity of the ‘‘ Pearl-sides ” (Mawrolicus pennanti, Cuv. 
and Val.) of our own shores, though from its wide distribution 
this lack of information seems to be remediable. 
In recent times phosphorescence has generally been associated 
with deep-sea fishes. Thus in a narrative of the early part of the 
voyage of the Challenger (NATURE, August 28, 1873) Sir Wyville 
Thomson mentions ranges of spots or glands producing a phos- 
phorescent secretion on the body of a fish pertaining to the 
Sternoptychidz, a species of which is included by |r. F. Day in 
the British list. Of a new Zchiostoma (one of the Stomiatidz) 
it is also noted that the two rows of probably phosphorescent 
dots along the body were red, surrounded by a circle of pale 
violet (‘* Challenger Narrative, Zoology,” I. vol. ii. p. 42). Dr. 
Giinther (‘‘ Challenger Narrative, Zoology, I. part ii. p. 905) 
observes that many deep-sea fishes have round, shining, mother- 
of-pearl bodies embedded in the skin. These are supposed to be 
producers of light, and they have been observed to be phos- 
phorescent in two species of Sternoptychide. He further 
states that the whole muciferous system is dilated in deep-sea 
fishes, that is, fishes inhabiting 1000 fathoms or more, and that 
the entire body seems to be covered with a layer of mucus, the 
physiological use of which is unknown; it has been noticed to 
have phosphorescent properties in perfectly fresh specimens. 
Having thus briefly reviewed the leading features of phos- 
phorescence in marine animals, a glance may now be taken at 
the supposed causes and purposes of this provision. 
I do not deem it necessary to go into detail with regard to the 
numerous views which have been advanced to account for the 
phosphorescence of marine organisms, for these range over a 
very wide area—from its production by electricity, the constant 
agitation of the water, by putrefaction, by luminous imbibi- 
tion, to its manifestation as a vital action in the animals, 
or a secretion of a phosphorescent substance. Ehrenberg con- 
sidered it a vital act similar to the development of electricity, 
and sometimes accompanied by the secretion of a mucilaginous 
humour which is diffused around ; while others, such as Meyen, 
thought it only a superficial oxidation of the mucous coat, or a 
luminour secretion from certain glands. Some believed that a 
liquid containing phosphorus was secreted, and that this under- 
went slow combustion ; while others explained that it was a 
nervous fluid modified by certain organs to appear as light. 
Coldstream thought it was due to an imponderable agent, and 
that phosphorus or an analogous substance might enter into the 
organs producing it. De Quatrefages, again, clearly affirms that 
it is produced in two ways: (1) by the secrction of a peculiar 
substance exuding from the entire body or a special organ; and 
(2) by a vital action independent of all material secretion. 
Panceri was strongly impressed with the importance of fatty 
matter in the forms he examined—such as Fenmatula, the 
Medusze, Beroides, Pholades, Chetepteri, and Noctiliu-e—the 
phosphorescence arising from the slow oxidation of this sub- 
stance ; the nervous system of the living animal, however, being 
capable of producing a momentary oxidation more rapid and 
more intense, accompanied by light. 
It will be observed that in the Protozoa the structure of the 
minute but often very abundant animals which furnish the 
luminosity clearly proves that the presence of a well-defined 
nervous system is not required for its manifestation, the proto- 
plasm of their bodies alone sufficing for its development. There 
are neither glands for secreting it, and in some apparently no 
fatty matter for slow combustion. In the Ccelenterates the 
phenomena appear to be more nearly related to nervous mani- 
festations, though in certain cases the luminous matter possesses 
inherent properties of its own. While in some annelids, 
such as Chetopterus and Polyirrus, there are glands which may 
be charged with the secretion of a luminous substance, it is 
otherwise with certain Polynoide, in which the emi:sion of 
light appears to be an inherent property of the nervous system. 
The irritability in the phosphorescent examples of the latter 
family, however, varies considerably, some, ¢.g. Polynoé scolop- 
endrina, being sluggish, while others, like Harvmothoé, are 
extremely irritable. In the Crustaceans the Inminosity seems to 
have the nature of a secretion, probably under the control of the 
nervous system. In Pyvosoma and Pholas dactylus a luminous 
secretion is also a prominent feature, and in both the latter and 
the annelids decay excites its appearance, as also is the case, to 
a limited extent, in fishes. 
It is evident, therefore, that the causation of phosphorescence 
is complex. In the one group of animals it is due to the produc- 
tion of a substance which can be left behind as a luminous trail. 
The ease, for instance, with which in Pexnafula and other 
Ccelenterates the phosphorescence can be repeatedly produced 
by friction on a surface having a minute trace of the material, 
clearly points to other causes than nervous agency. The action, 
moreover, clearly affects the organic chemical affinities of the 
tissues engaged. On the other hand again, as in certain 
annelids, it is purely a nervous action, probably resembling that 
which gives rise to heat. 
With the exception of such as Macartney, the older authors, 
who in some cases took an imaginative view of the question, 
connected the emission of light with the special economy of the 
deep sea. The speculations to this effect are fairly summarised 
in ‘‘ Brewster’s Edinburgh Encyclopedia,” published in 1830 
(Chiefly the views of Dr. Macculloch). Thusit is supposed that 
total darkness exists at the depth of tooo feet, and that the phos- 
phorescence of marine animals is a substitute for the light of the 
sun. Moreover, that by these lights the animals on the one 
hand are guided for attack, and on the other their power of 
extinguishing them enables them to escape destruction. Fishes 
are known to prey chiefly at night, and the writer supposes that 
the phosphorescence of their prey guides them; for, he says, 
this luminosity is particularly brilliant in those inferior animals 
which from their astonishing powers of reproduction, and from a 
state of feeling little superior to that of vegetables, appear to 
have been in a great measure created for the food of the more 
perfect kinds. Dr. Coldstream at a later period (1847) re- 
produced the same views in his article on animal luminosity 
(Todd’s ‘*Cyclop. of Anat. and Phys.”). 
The same notion was brought forward in the ‘‘ Report of the 
Cruise of the Porcupine” (Proc. Roy. Soc., No. 121, 1870, 
p- 432), and special reference was made to the young of certain 
starfishes, which are stated to be more luminous than the adults, 
that being part of the general plan which provides an excess of 
the young of many species, apparently as a supply of food, their 
wholesale destruction being nece sary for the due restriction of 
the multiplication of the species, while the parent individuals, 
on the other hand, are provided with special appliances for 
escape or defence. Thus phosphorescence, it is further 
asserted (‘‘ Depths of the Sea,” p. 149), im very young 
Ophiacanthee just rid of their plutei, in a sea swarming with 
predaceous crustaceans, such as Dorynchus and Munida, with 
great bright eyes, must be a fatal gift. Some naturalists still 
appear to hold a similar, though perhaps modified view. Much 
caution, however, is necessary in theorising on this head. 
In the first place, phosphorescent animals do not appear to be 
more abundant in the depths of the sea than between tide-marks 
or on the surface, the latter perhaps presenting the maximum 
development of those exhibiting this phenomenon. Very many 
of the young that have been indicated as so brilliantly luminous 
become surface-forms soon after leaving the egg, and thus at 
their several stages more or less affect the three regions—of 
surface, mid-water, and bottom. 
A survey of the life-histories of the several phosphorescent 
groups affords at present no reliable data for the foundation of a 
theory as to the functions of luminosity, especially in relation 
to food. No phosphorescent form is more generally devoured 
by fishes or other animals than that which is not; and, on the 
other hand, the possessor of luminosity, if otherwise palatable, 
does not seem to escape capture. An examination of the 
stomachs of fishes makes this clear, except perhaps in the case 
of the hersing, which, however, is chiefly a surface-fish. 
Further, it is not evident that such animals are luminous at all 
times, for it is only under stimulation that many exhibit the 
phenomenon. 
Moreover, the irregularity of its occurrence in animals pos- 
sessing the same structure and habits in every respect, strengthens 
the view just expressed. Thus, while P/o/as dactylus has been 
known from the days of Pliny to be luminous, the common 
Pholas crispata is not so endowed. Two annelids abound 
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