Some animals possess a periodicity of luminescence. They only luminesce at night and fail to respond to stimulation or are difficult to stimulate during the day. Bright light has an inhibiting effect. Perhaps correlated with this is the fact that most luminous forms are strongly negatively heliotropic. Fireflies lie hidden in the day, to appear about dusk and the ostracod crustacean, _Cypridina_, is difficult to obtain on moonlight nights. The Ctenophores were the first forms in which the inhibiting effect of light was noticed. This was described by Allman (1862) and has been confirmed by a number of observers, especially Peters (1905). Massart found that _Noctiluca_ was difficult to stimulate during the day and _Ceratium_, according to both Zacharias (1905) and Moore (1908), only luminesces at night, or if kept in darkness, for some little time. Crozier[4] finds a persistent day-night rhythm of light production when _Ptychodera_, a balanoglossid, is maintained for eight days in continued darkness. The animal is difficult to stimulate during the period which corresponds to day and luminesces brilliantly and at the slightest touch during the period which corresponds to night. On the other hand, a great many forms are able to luminesce quite independently of previous illumination. According to Crozier[4] _Chætopterus_ luminescence is not affected by an exposure to 3000 metre-candles for six hours. [4] Private communication. In the case of animals with extracellular luminescence we may speak of luminous secretions and true luminous glands. A large number of forms possess luminous glands or gland cells, including some of the _medusæ_, the hydroids (probably), the pennatulids (?), the molluscs (_Pholas_ and _Phyllirhoë_) (probably), some cephalopods (_Heteroteuthis_ and _Sepietta_), most annelids, ostracods, copepods, some schizopods (_Gnathophausia_) and decapod (_Heterocarpus_ and _Aristeus_) crustaceans, all myriapods, and the balanoglossids. The remaining organisms burn their material within the cell. These include the bacteria, fungi, protozoa, some medusæ (?), ctenophores (probably), most cephalopods, a few annelids (_Tomopterus_ (?)), ophiuroids (?), some schizopod (_Nyctiphanes_, _Euphasia_, _Nematocelis_, _Stylochiron_) and decapod (_Sergestes_) crustacea, all(?) insects, _Pyrosoma_, and fishes (_selachians_ and _teleosts_). It is among this latter type that the most complicated luminous organs have been developed. While a description of all the types of luminous organs and luminous structures cannot be attempted here (excellent descriptions have been given by Dahlgren and Mangold) it is necessary to understand the structural conditions in a few of the forms whose physiology has attracted most attention.