Luminescence in decapod crustaceans may be of two kinds. Either a secretion is ejected into the surrounding water as a luminous cloud, or specific light organs, which may be internal or superficial, glow steadily (Balss, 1944). There may be more than 150 photophores in some species of decapod, and the number, position and development of these organs in Systellaspis debilis and species of Sergia have been described by Coutière (1906), Kemp (1910a), Terao (1917) and Gordon (1935). The production of luminous clouds has been observed in the Oplophoridae, Pandalidae and Thalassocarididae of the Caridea, and in the Penaeidae of the Penaeidea (Table 1). Observations of the luminescence of specific light organs or tissues are much less numerous. Luminescence has been observed among the Oplophoridae from the photophores of species of Systellaspis and Oplophorus and from the liver of Hymenodora. The hepatic organs of Parapandalus of the Pandalidae have been seen to luminesce, and among the Penaeidea both the hepatic organs of Pesta of Sergestes and the superficial organs of Sergia have been seen to emit light (for references see Table 1). Undoubted hepatic light organs also occur in Chlorotocoides and Thalassocaris of the Thalassocarididae, and superficial organs in Hymenopenaeus debilis of the Penaeidae. Additional organs of doubtful function which have been described as possible photophores occur on the chelae of the homolid crab Hypsophrys, in the branchial chamber of Sergestes, the pleopods of Parapandalus and the uropods of Acetes (Table 2). There are unconfirmed reports of luminescence in the eye of Pasiphaea etarda (Kiernik, 1908) and in Polycheles (Alcock, 1902) and Leptochela (Chace, 1940), neither of the latter two cases being ascribed to a particular organ, though ‘a greasy glandular patch’ near the oviduct was implicated in Polycheles.