The sea-pen _Pavonia_[20] is noted for its light-emitting properties; and during the voyage of the English ship “Porcupine” the naturalists on board had many opportunities for observing its display. Sir Wyville Thompson, who was in charge, says, “Coming down the sound of Skye from Loch Torridon on our return, we dredged in about one hundred fathoms; and the dredge came up tangled with the long pink stems of the singular sea-pen. Every one of these was embraced and strangled by the twining arms of an _Asteronyx_,[21] and the round soft bodies of the star-fishes hung from them like plump ripe fruit. The _Pavonariæ_ were resplendent with a pale lilac phosphorescence, like the flame of Cyanogen gas; not scintillating like the green light of some sea-stars,[22] but almost constant, sometimes flashing out at one point more brightly, and then dying gradually into comparative dimness, but always sufficiently bright to make every portion of a stem caught in the tangles or sticking to the ropes distinctly visible. From the number of specimens of sea-pens brought up at one haul, we had evidently passed over a forest of them. The stems were a meter long (over three feet) fringed with hundreds of polyps.” When the ship “Venus” was lying off Simonstown, one of their boats passed over a forest of sea-pens in shoal water, which gave out a vivid light; while, where the ship lay at anchor, other forms of phosphorescent animals illuminated the ports so that the men lay in them and read by the wondrous light on the darkest night. The _Renilla_[23] is a rich purple species common on our south-eastern borders. Agassiz found it at Charleston, S.C., and says of its phosphorescence, that “it emitted a golden-green light of wonderful softness.” _Virgularia_[24] is an attractive form; and in certain portions of the Patagonian coast they have been seen, when left by the tide, emitting a light of great brilliancy. _Vertillum_ is an interesting genus, resembling a quill pen in which the feathers have been curled or singed. Its color is a brilliant orange; but in the darkness it develops a phosphorescence of great beauty, and so penetrating that a glass containing numbers of them has been used as a lamp to read by,--an interesting example of one of the possible, though not remarkably practical, uses of living lights to mankind. [Illustration: PLATE VIII. LUMINOUS STAR FISHES. (_Brisinga elegans._) From 4,500 feet deep.] CHAPTER IV. LUMINOUS ECHINODERMS.