In _Cypridina hilgendorfii_ the luminous gland is situated on the upper lip near the mouth. It is made up of elongate (some 0.7 mm. in length), spindle-shaped cells, each one of which opens by a separate pore with a kind of valve. The openings are arranged on five protuberances. Muscle fibres pass between the gland cells in such a way that by contracting the secretion can be forced out. In the sea water the secretion luminesces brilliantly and the Japanese call these forms _umi hotaru_, or marine fireflies. Fig. 22 is a diagram showing the structure. Watanabe (1897), who first studied this form, and also Yatsu (1917) have described two kinds of granule-containing cells, one with large yellow globules, 4-10µ in diameter (Fig. 23), the other with small colorless granules 0.5, in diameter. I have observed in the living form these two types and also large colorless globules of the same size as the yellow globules. All dissolve when extruded into the sea water. Dahlgren[5] has described from sections four types of cells containing (1) large globules, (2) small granules, (3) a fat-like material, (4) a mucous material. Just what the significance and nature of these types of substance is cannot be stated at present. At least one, probably two, are concerned in light production. The others may possibly form digestive fluids which act on the food of the animal. [5] Private communication soon to be published. Turning now to the animals possessing light cells with intracellular luminescence we find in general that such light cells are localized to form definite light organs and that these may be single, as in the common fireflies, paired, as the prothoracic light organs of _Pyrophorus_, or scattered over the surface of the body, as in so many shrimps, cephalopods and fishes, when they are often called photophores. The light cells proper are often associated with reflectors, lenses, opaque screens and color screens. [Illustration: FIG. 24.--Distal portion of malpighian tubule of _Bolitophila_, showing modification to form photogenic organ (_after Wheeler and Williams_). _MT_{1}_, _MT_{4}_, malpighian tubules forming photogenic organ; _R_, reflector; _M_, muscle; _T_, trachea.] The insects possess the simplest types of intracellular light organs, a mass of photogenic cells, which, in the common firefly (_a lampyrid beetle_) of Eastern North America, has probably been developed from the fat body, while in the New Zealand glowworm, the larva of a tipulid fly (_Bolitophila luminosa_), part of the Malpighian tubule cells have acquired photogenic power (Wheeler and Williams, 1915). This is illustrated in Fig. 24.