Chapter VI

) is the only one definitely known. Perhaps we may place in

this category also the instances of photoluminescence, but the chemical reaction involved cannot be pointed out.

We know of no animal whose eyes, the organs, _par excellence_, of photochemical change, give off light in the dark. All cases of luminous eyes have been conclusively shown to be purely reflection phenomena. The eyes of a cat only glow if some stray light is present which may enter and be reflected out again. Photochemical reactions and chemiluminescent reactions do have this in common, however, that they are largely but not exclusively oxidations. Whether all photochemical changes in the eyes in animals require oxygen or not, is unknown, but all animal light-producing reactions, without exception, are oxidations, and light is only produced if oxygen is present. Some material is oxidized.

In general, we may divide luminous organisms into two great classes according as the oxidizable material is burned within the cell where it is formed or is secreted to the exterior and is burned outside--intracellular and extracellular luminescence. Many animals with intracellular luminescence have quite complicated luminous organs. It is an interesting fact that a great similarity may be observed between the evolution of the complex organs of vision and of these complicated organs. In the simplest unicellular forms certain structures within the cell serve as the photochemical detectors of light, while in luminous protozoa, similarly, granules scattered throughout the cell are oxidized with light production. In the higher forms the eye contains groups of photosensitive cells connected with afferent nerves, lenses, and accessory structures for properly adjusting the light, while luminous organs contain groups of photogenic cells in connection with efferent nerves, lenses, and accessory structures for properly directing the light. It is interesting to note that in the two groups where the eye has attained its highest development, the cephalopods and vertebrates, here also the luminous organ is found in greatest complexity and perfection. In intermediate stages of evolution the eye and luminous organ so closely approach each other in structure that it is still a mooted question whether certain organs found in worms and crustacea are intended for receiving or producing light.

We may also divide luminous forms into two groups according as the oxidation of luminous material goes on continuously, independently of any stimulation of the organism; or is intermittent, oxidation and luminescence occurring only as a result of stimulation, using the word "stimulation" in the same sense in which it is used in connection with nerve or muscle tissue. Bacteria, fungi, and a few fish produce light continuously and independently of stimulation. Its intensity varies only over long periods of time and is dependent on the nature of the nutrient medium or general physiological condition of the organism. All other forms give off no light until they are stimulated. Stimulation may of course come from the inside (nerves) or outside. Only under unfavorable conditions, such as will eventually lead to the destruction of the luminous cells, do these forms give off a continuous light. This has often been spoken of as the "death glow," and is to be compared with _rigor_ in muscle tissue.

Some of the fish which produce a continuous light possess a movable screen similar to an eyelid which can be drawn across the organ, thus shutting off the light, so that the animal appears to belong to the group which flashes on stimulation. This is true of _Photoblepharon_, while _Anomalops_ can rotate the light organ itself downward, so as to bring the lighting surface against the body wall and thus cut off the light (Steche, 1909). Other fish (_Monocentris_) are unable to "turn off" their light.

Animals which flash spontaneously on stimulation through nerves from within, possess a very varied rhythm. The different species of fireflies can be distinguished by the character of their flashing (McDermott, 1910-17; Mast, 1912). Fig. 16 shows the method of flashing of some common eastern North America species. The glowworm light lasts for many seconds and then dies out. This interval of darkness persists for some minutes and is then followed by another period of glowing. Some fireflies have a light which may be described as partially intermittent. It lasts for hours, but may become more dim or be intensified on stimulation.

[Illustration: FIG. 16.--Chart showing relative intensities and durations of flashes of American fireflies (_after McDermott_). One cm. vertically = approximately 0.02 candle power; one cm. horizontally = approximately one second. The flash of the males ([male]) is at the left; that of females ([female]) at right of chart.]

Some forms only produce light at certain seasons of the year. According to Giesbrecht (1895) this is true of the copepods, which only light in summer and autumn, and according to Greene (1899) in the toad-fish; _Porichthys_, which can only be stimulated to luminesce during the spawning season in spring and early summer.

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] _Chaetopterus_ 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 _medusae_, the hydroids (probably), the pennatulids (?), the molluscs (_Pholas_ and _Phyllirhoe_) (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 medusae (?), 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.

Luminous bacteria are so small that the light from a single individual cannot be seen. It is almost impossible to make out structural differences within the cell and we cannot definitely state in just what special region, if any, the luminescence is produced. We do know that the light is intracellular and that filtration of the bacteria from their culture medium gives a dark sterile filtrate absolutely free from any luminous secretion.

Among protozoa, in certain forms at least, it is easy to observe that luminescence is connected with globules or granules which were considered by the earlier observers to be oil droplets. Thus, in _Noctiluca_ (Figs. 17 and 18), when the animal is violently stimulated or in the presence of reagents which slowly kill it, the whole interior appears a mass of starry points of light which can be traced to minute granules along the strands of protoplasm (Quatrefages, 1850).

[Illustration: FIG. 17.--_Noctiluca miliaris_, showing photogenic granules in cytoplasm. _n_, nucleus; _c_, cytoplasmic strands containing photogenic (large) and other (small) granules; _p_, pharynx; _f_, flagellum; _o_, oral groove; _t_, tentacle; _s_, spines at base of tentacle; _v_, vacuoles. _Drawn by E. B. Harvey._]

[Illustration: FIG. 18.--_Noctiluca miliaris_ as it appears during luminescence (_after Quatrefages_). Upper left and middle, low power; below, high power; upper right, a crushed fragment still luminescent.]

Turning to the multicellular forms, we find the simplest development of luminosity in those animals which possess gland cells producing a luminous secretion. These cells may be scattered over the surface of the animal as in _Chaetopterus_ (Fig. 19) or _Cavernularia_, or restricted to certain areas [_Pholas_, (Fig. 19),] or more definitely localized to form an isolated group of gland cells as in _Cypridina_. True multicellular glands also occur. In every case, however, we find that the luminosity of these uni- or multicellular glands is connected with the presence of granules. They are often spoken of as _luciferine granules_, although it is not certain whether they are made up of luciferin or luciferase (see