CHAPTER VI.
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.]