6. Immersion in vacuo.—When glow-worms are placed in vacuo, their light fades, but re appears on admission of air.
7. Removalji-oni alljbreign sources of light. —If luminous insects be confined in a dark place, they shine little in the early part of the day, but long before night they begin to do so; although generally, in their native situations, they do not emit light until the twilight. If the confinement in a dark place be protracted, they do not shine so brightly as after having seen the sun during the day.
I V. Seat of luminousness in different ali4.1s. —In most of the luminous animals that inhabit the ocean, a great part of their surface seems to be endowed with the property of forming, and pouring out, a mucous fluid, which contains the luminous matter, and is frequently mis cible with water and other fluids. This some times so entirely covers the whole animal as to cause it to emit light from every point of its surface; but more generally when the animal is swimming, the light is seen to proceed only from certain regions. Some of the medusa., even of the largest size, emit light from a very small point, particularly when the luminous organ is placed in the central parts of the body. When the light is vivid, it seems to be larger than it really is, from the refracting power of the gelatinous tissues through which it passes. Occasionally the luminous point has not a diameter equal to the 1-200th of that of the animal itself. In cydippe pileus and oceania pileata of the Baltic, Ehrenberg finds that the light issues solely from the vicinity of the ovaries, and in Oceania hemispherica, from the bases of the cirri. Photo dactylics gives out light most strongly from the internal surface of its respiratory tubes. The luminous mucus is sometimes poured out even by very small animals in such quantity as to leave a lumi nous wake behind them, as in an instance mentioned by Quoy and Gaimard. These ob servers saw such luminous lines formed in the paths of certain extremely small creatures, so transparent that their forms could not be dis tinctly made out. The positions of their bodies
were marked in the water by bright spots, which were followed in their course by lumi nous wakes, at first about an inch in breadth, but afterwards by the movements of the water spread out to the breadth of two or three inches. This luminous mucus is supposed to be the seat also of the remarkable stinging property possessed by many of the acalephce. It retains its luminousness in some instances for a day or two after being emitted by the animal, but loses it whenever putrefaction commences.
But although this luminous mucus be so ge nerally secreted and emitted by marine ani mals, it is evident that the light given out by many of them has its seat in certain organs more or less internal, whence it proceeds in gleams and momentary flashes that seem to depend only on the movements of some im ponderable agent. The exact position and re lations of these organs can seldom be satis factorily discovered, but in some crabs and mi nute crustaceous animals that emit light, it is observed to proceed from the central organs of the nervous system. In other crustacea the whole body seems to be full of light, which is emitted, as at so many windows, through the translucent membranes interposed between the segments of the crust. Dr. Macculloch con eludes from his numerous observations on this subject, that, in marine animals generally, the coats of the stomach and intestines are the light-giving organs.
In insects the seat of luminousness is more satisfactorily ascertained, and is found to vary very much in different species and tribes. The eggs of the lampyrides are said to be fre quently seen luminous, and to continue so for several days after being deposited. In the states of larva and chrysalis also, the same in sects emit light most vividly when touched, chiefly from the posterior segments of the body. On being much irritated, the whole of the chrysalis seems to shine in a slight degree, and for a short time.