In the Arctic regions beautiful lights have often been seen, due to a minute crustacean. Lieut. Bellot first observed it in the North-American polar regions, and Nordenskiöld refers to it in his “Voyage of the Vega.” The most brilliant displays have been seen at Mussel Bay. Nordenskiöld says, “If during winter one walks along the beach on the snow, which at ebb is dry, but at flood-tide is more or less drenched through with sea-water, there rises at every step an exceedingly intense beautiful bluish-white flash of light, which in the spectroscope gives a one colored labrador-blue spectrum. This beautiful flash of light arises from the snow, that shows no luminosity before it is stepped upon. The flash lasts only a few moments, but is so intense that it appears as if a sea of fire would open at every step a man takes. It produces, indeed, a peculiar impression on dark and stormy winter days. The temperature of the air is sometimes in the neighborhood of freezing of mercury. It is certainly a strange experience to walk along in this mixture of snow and flame, which at every step one takes splashes about in all directions, shining with a light so intense that one is ready to fear that his shoes or clothes will take fire. If carefully examined, the cause of this phenomenon is found to be a little crustacean, _Metridea armata_, that somewhat resembles the Cyclops. The great changes of temperature to which it is subjected in the snow-sludge seem not to affect it.” Few phosphorescent animals exhibit their glories during the day; but _Sapphirina_ (Plate X., Fig. 6) is an exception. It is one of the largest of the _Entomostracans_, about a quarter of an inch in length, broad and flat, without the beauty of form which characterizes Cyclops, Calanus, and others; but what it lacks in this respect is more than compensated by its marvellous powers of light production, few animals of any kind equalling it. So vivid is the phosphorescence, that it can be distinctly seen by day; and, peering down into the depths where it abounds, flashes of color--blue, gold, sapphire, purple, green, and other hues--appear in bewildering frequency, ranging from the softest to the most intense and vivid lights, marking this living sapphire as one of the true gems of the sea. [Illustration: PLATE XIV. SPIDER CRAB. (_Colossendeis._)] Giglioli mentions an Isopod crab, brilliant with gold and purple, gorgeous with iridescence, and possessed also of the additional charm of phosphorescence. The light-emitting organs in the _Entomostracans_ observed by him were in the anterior portion of the thorax.