The young (Zoëa) of the graceful little opossum shrimp _Mysis stenolepis_ is phosphorescent. The adult forms are extremely interesting objects for study, the eggs and young being carried in a little pouch beneath the thorax. Allied to this little sea-opossum is _Lucifer_, that is to the crustaceans what the walking-stick is to the insect world; a veritable incongruity, resembling a branch of weed, and doubtless finding some protection in the mimicry. Some specimens, according to Giglioli, are luminous; the gift perhaps forming a signal language, a code understood in this world under the sea. The position, or seat, of the luminosity in crustaceans differs as widely as the intensity and color of the light; and in the little _Stomatopod_, formerly considered as an adult, and described as _Squillerichthus_, we find the culmination of wonders, as, in a specimen of this genus found in the Atlantic, the seat of the brilliant intermittent yellowish-green light is in the eye-stalk; so that the eyes themselves may be said to be veritable lanterns. The phosphorescence of crabs was probably observed for the first time by Sir Joseph Banks, on his voyage from Madeira to Rio Janeiro; a small crab, named _Cancer fulgens_, being captured, which was remarkably luminous. Sir Joseph does not state whether the light came from the entire body or was confined to certain localities. MM. Eydoux and Souleyet, naturalists of the French exploring ship “La Bonite,” noticed a small luminous crustacean, and succeeded in separating the phosphorescent secretion from the animal. They describe it as yellowish, viscous, and soluble in water, and found that its luminous properties soon disappeared. It was their opinion that certain crustaceans secreted the luminous matter, and that they differed much in their method of producing it. Certain small crabs, they believed, could display a certain amount of light when irritated; the phosphorescence at these times appearing in jets, forming a cloud or halo of light in which the animal seems to disappear. In the abyssal depths of the ocean, where probably no ray of sunlight reaches, the crabs are possibly all luminous. Many of these deep-sea forms have a wide geographical distribution. Thus the Lithodes are found from the shallow waters of the north and south poles to the tropics, in the latter living in a region over which rests three-quarters of a mile of water. Many other crustaceans live in depths vastly more inaccessible than this, and under a much greater pressure. Thus _Colossendeis titan_, a strange creature, whose stomach is prolonged to the ends of the feet, is found living at a depth of about two miles and a half. These creatures, a species of which is shown in Plate XIV., are the spiders of the sea, resembling their not distant allies of the land, at least in appearance.