LAMP SHELLS.
In all the forms previously mentioned, the phosphorescence is
conspicuous; but in the little bivalve Pholas it is almost hidden.
The shells of the family _Pholadidæ_ are noted for their boring
habits; penetrating into the hardest stone, as granite and gneiss,
literally entombing themselves, as shown in Plate VII., Fig. 1, which
represents a section of a block of granite into which the little
animals have penetrated. How they can perform such a work, is something
of a mystery; but the foot, which is provided with a hard dermal
protection, is probably the instrument used by the miner.
The most remarkable evidence of their work, according to
Figuier,--though it is fair to say he has been disputed,--is seen
in the Temple of Serapis on the Pozzuolan coast, where the pillars
are perforated with holes, which this author claims were made by the
Pholas,[35] when by a sinking of the crust the pillars were under
water; the columns, by a reverse motion, having now re-appeared from
the sea, bearing the evidences of their submersion.
As if to still further carry out the idea of the miner, the animal
bears its own light, which, though vivid, could but little more than
illumine the stony prison into which the Pholas has willingly ensconced
itself. In Borneo, a freshwater form has been found boring in the dead
trunks of trees. Pliny was probably among the first to place on record
the luminosity of this little borer, having stated that it shone in the
mouths of those who ate it; and its phosphorescence has been studied by
Réaumur, Beccaria, Marsilius, Galeatus, Montius, and others in modern
times. One of Beccaria’s experiments was to ascertain how the light
affected certain colors. He secured a Pholas in a dark spot, and placed
in its light ribbons of various colors. The white ribbon shone most
brilliantly, the yellow next, and the green next, while others were so
indistinct as to be hardly noticeable. Substituting liquids for the
ribbons, the result was the same.
Beccaria also made one of the first practical applications of the
phosphorescent Pholas, demonstrating that it could be used as a lamp.
This was accomplished by placing one in seven ounces of milk, which
rendered the latter so luminous that print could have been read by it,
the milk appearing almost transparent. So it is within the bounds of
possibility to write a _post-mortem_ description of the Pholas by
its own light.
It is evident from these simple experiments that the discovery of
the secret of phosphorescence, and its practical application to the
wants of mankind, would result in revolutionizing present systems,--a
heatless, inexpensive, unextinguishable light being the perfection of
possibilities in this direction,--and it is not improbable that the
experimentalists of olden times may have had this in view when making
their investigations. Both Réaumur and Beccaria attempted to render
the light of this animal lamp permanent. By placing one in honey, the
luminosity was apparently preserved for a year, the light re-appearing
whenever the mollusk was placed in warm water. Brandy extinguishes
the light, and Galeatus and Montius found that vinegar and wine
produced the same result. If the body of Pholas is heated slowly, the
light gradually becomes more and more intense, until, finally, at 45°
Réaumur, or 56° Centigrade, it disappears, and cannot be restored.
The secure position of the Pholas in its impregnable fortress would
hardly seem to require a warning or attractive light; and its use must
remain a mystery, though theory could, of course, suggest explanations.
While the Pholas conceals its luminosity in its dungeon, there are
other molluscan light-givers which float about like light-ships astray.
These are Pteropods, or wing-footed mollusks; delicate fairy ships of
marvellous beauty. By some authorities they are said to represent the
higher forms of the _Cephalophora_, while others consider them as
degenerate or backsliding Cephalopods, of which the squids and octopi
are representatives. They are pelagic, free-swimming mollusks, in which
portions of the foot are modified into seeming wings, so that the
little creature seems to fly through the water. They differ much in
appearance. Some secrete a glassy, horny, cartilaginous or limey shell,
which in some cases is only present in the larval forms, disappearing
in the adult; while others, again, preserve it through their entire
lives. The body is of various shapes: it is protected by the shell when
present, and can be drawn into it.
Though simple, helpless creatures, many have an armament which in
a larger animal would be considered extremely effective. Thus in
_Clio_ each tentacle bears nearly three thousand cylinders, each
containing stalked suckers; and, as there are six tentacles, the little
animal can grasp its microscopic prey with three hundred and sixty
thousand hands. Besides this, it has a pair of many-toothed jaws and a
toothed tongue. While extremely small, these animals exist in such vast
multitudes, that they probably constitute an important food for certain
whales.
[Illustration:
PLATE VI.
APOLEMIA. CLEODORA. PRAYA.]
One of the most interesting of the Pteropods, or wing-footed animals,
as associated with our present subject, is the _Cleodora lanceolata_
(Plate VI., Fig. 1). It has a pyramidal shell, terminating in three
sharp spines, the wing-like fins rising above. It is rarely over half
an inch in length, almost transparent, and bears in its shell a small
light, which, however, is distinctly seen through the transparent
covering. A more beautiful living lamp it would be difficult to
imagine; and when slowly flying through the ocean, in countless
myriads, they must present a wondrous sight. One of this genus,
observed by Giglioli, emitted a very livid red light; the luminous
organ being at the summit of the shell. There are many different genera
and species. _Hyalea_, an oceanic wing-foot, moves very rapidly,
and looks not unlike a butterfly darting here and there, in erratic
flight, in search of food; but the little _Cleodora_ moves in a regular
and stately manner. In _Hyalea_ observed by Giglioli in the harbor
of Anjer, Java, the light, which contributed largely to the general
phosphorescence, was confined to the basal part of the shell.
My young readers interested in geology are probably familiar with the
curious _Conularia_, or cone in cone, which has been found in Australia
sixteen inches in length, and has always been regarded a puzzle. It
has been suggested that this is a gigantic fossil Pteropod. The little
needle-like Tentaculites, from the Silurian and Devonian rocks, are
also allies.
Some of the most remarkable mollusks are found among the sea-slugs, so
called from their resemblance to the slugs of the garden. I have found
them on the weed floating in the Gulf Stream, so resembling the latter
in almost every particular that it was difficult to determine that they
were not a part of the weed itself. _Scyllaea pelagica_ is such a form;
helpless, yet finding protection in its mimicry of the surroundings.
Equally as remarkable is _Dendronotus_,[36] the bushy sea-slug whose
gills resemble the branches of weed in a remarkable manner. This
curious sea creature is quite common on the seaweeds of our New-England
beaches. In the Mediterranean and Pacific is found the most unique
of the group, the _Phyllirhoë bucephala_ (Plate V., Fig. 2), which
differs from many so entirely that it would seem to belong elsewhere.
Like the other forms, it is pelagic, often being seen swimming along,
resembling a fish, with its compressed body, and vertical, fan-like
tail, and with long feelers or tentacles ahead. It is transparent and
shelless in the adult stage, possesses no foot or _branchiæ_, evidently
breathing through the body-walls or general surface. To add to its
curious features, the _Phyllirhoë_ is brilliantly phosphorescent; light
being emitted from certain spots, shown in the engraving, rendering the
tissues transparent and luminous. Examination has shown that the light
proceeds from certain globular nucleated cells, which appear to be the
terminations of nerves.
The _Phyllirhoë_ thrives well in the aquarium, and has been studied
and observed in the famous aquarium at Naples. When it is touched or
is swimming, the light seems to diffuse the entire surface, so that it
presents a striking contrast against the dark water; and undoubtedly
this gift is a fatal one, attracting the attention of many a fish to
the dainty morsel seemingly outlined in fire.
Giglioli refers to the luminosity of an undescribed Heteropod, the axis
of whose body gave out a reddish light whenever the animal was excited.
According to C. W. Peach, the young of _Eolis_ are phosphorescent.
Such instances where the animal is particularly defenceless are amusing
refutations of the theories of naturalists who see in the light a
warning.
The common garden slugs, the cousins of the snails, are well-known
forms. They generally remain concealed during the day, coming out at
night, and often doing much damage to vegetation which is largely laid
to birds. I have kept many of them, and they offered an extremely
interesting study. They secrete a remarkable amount of mucus, which
they use in descending from a tree, just as a spider does its silk
thread. The mucus exudes from the foot, passes along to the tail, when
it is attached to the twig. This accomplished, the slug boldly launches
itself into space, the thread becoming more and more attenuated, until
finally, when the slug is near the ground, it is exceedingly fine.
Nearly all our common slugs descend from trees in this manner,--quite a
contrast to the slow, tedious ascent.
The amount of mucus that can be taken from them is remarkable; and that
it is also protective will be evident to any one who may experiment
with them.
One genus, _Phosphorax_, found at Cape Verde, and, according to
Duncan, at Teneriffe, has a luminous pore on the posterior border
of the mantle. One species only is known, _P. noctilucus_; and its
light has not, that I am aware, been made the object of any extended
investigation.
The highest forms of the _Mollusca_, the Cephalopods, cuttle-fishes,
are probably at times luminous. I have noticed what I presumed
was a delicate, sensitive, luminous glow about an _Octopus_ in a
semi-darkened tank, but I am not satisfied to make the statement as
fact. These forms are so remarkable for the waves of color that pass
over them, and which seem to make them transparent, that one could
readily be deceived.
The little _Cranchia_ (Plate IV., Fig. 2) is a light-giver, its
phosphorescence having been distinctly observed. It is an ally of
the giant squids, which have been found fifty-five feet in length,
and which, if luminous like their pygmy relative, would present a
marvellous spectacle, darting veritable living arrows through the
depths of the sea.
Giglioli refers to the phosphorescence of _Loligo saggitatus_,
and to that of several small Octopods observed by him at Callao and
Valparaiso. Their bodies gave out a pale whitish light, uniformly
distributed.
[Illustration:
PLATE IX.
_c_ _b_ _a_ _f_ _e_ _d_
LUMINOUS BEETLES, ETC.
_a._--_Lampyris splendidula_--male.
_b._-- „ „ --female.
_c._-- „ „ --larva.
_d._--_Lampyris noctiluca_ --male.
_e._-- „ „ --female.
_f._-- „ „ --larva.]