If we could descend into these depths, we would find a veritable forest, with branches seeming on fire; many of the coral trees being from ten to fifteen feet in height, and equally as wide, forming lanes and open pathways through which the fishes pass, bathed in the wondrous light. That this is not imagination is shown by the branches brought up accidentally and by dredges; some limbs alone being four feet in length, and stout in proportion. Specimens of these forms can be seen in most of our museums of science; some sent by Dr. Holder from the Florida reef to the Museum of Natural History, in Central Park, are surprisingly beautiful. More remarkable than the Gorgonias are the strange animals belonging to _Pennatulidæ_,[18] known popularly as the sea-pens (Plate VII., Fig. 2), from the resemblance of some to a quill pen,--an abnormally large one, it must be confessed. One of the most familiar forms is _Pennatula phosphorea_. When the animal is observed at night, and disturbed, it emits quite a brilliant light. In specimens observed at Oban by Professor Marchel, the more perfect females became vividly phosphorescent when the leaves were gently irritated. When the polyps were touched, they showed minute points of light, which appeared over the whole surface, in rapid, irregular coruscations. If one of these living pens can produce so interesting a display, what must be the sight upon the bottom, where myriads of these curious forms abound, either fixed or moving! It is not impossible that the light-emitting faculty of sea-pens is under control; at least, they have their periods of darkness and light. If a specimen which is not luminous is disturbed, as we have seen, it immediately becomes so. If the long axial stem is pinched, a seemingly protesting light appears on the lowest branchlets nearest the stem, quickly spreading, as if the polyps were igniting. When all those on a branch have become luminous, the light begins to appear on the next, and so on in succession until the whole glows brilliantly. Four-fifths of a second occur between the stimulation and the appearance of the light; so that in a sea-pen six and one-tenth inches in length, two seconds and a fifth were required for its complete illumination. By pinching the top or opposite end of the colony, the same phenomenon resulted, but reversed. If a polyp at the end of a branchlet was irritated, light immediately appeared, passed to its neighbor, and so on; if a branch was touched at both ends, the light followed the act, and met in the centre. These interesting experiments, which were made by Panceri, can be varied in many ways by those fortunate in securing a live sea-pen.[19]