Chapter IV

) or both. They are most similar to the zymogen granules found so abundantly in gland cells and thought to be the precursors of various enzymes. According to Dahlgren (1915), the luciferine granules stain blue-black by iron haematoxylon after fixation at the boiling point, and photogenic cells can be detected by this method of selective staining. Dubois (1914, book), who regards them as examples of _bioprotein_, comparable to the chondriosomes and handed on from one generation to another, gives them the name of _vacuolides_ or _macrozymases_. In some forms he has described their transformation into crystals and believed at one time that animal light was a crystalloluminescence. His figures of the crystal transformation are not very convincing. Pierantoni (1915) has considered the granules to be _symbiotic_ luminous bacteria, but this is certainly not the case.

[Illustration: FIG. 19.--Diagram of _Pholas_ (right) and _Chaetopterus_ (left) to show distribution of luminous areas (_after Panceri_).]

The light of _Chaetopterus_ comes from a material mixed with a mucous secretion formed over almost the whole body surfaces of the animal. A section of the epithelium shows large mucous-producing cells and smaller granule-containing light cells (Fig. 20). These appear to be under nervous control, as a strong stimulation in one part of the body causes luminescence which spreads over the whole surface of the worm. The animal becomes fatigued rather readily, however. In the pennatulids, such as _Cavernularia_, we have also the formation of a luminous secretion over the whole surface of the body and the individual animals in this colonial form are also connected with nerves. A stimulation in any local region, as Panceri (1872) first showed (Fig. 21), will cause a wave of luminosity to spread from this point until it extends over the whole surface of the colony. In _Pennatula_ the rate of this luminous wave is about 5 cm. per second.

[Illustration: FIG. 20.--Sectional view of the luminous epithelium of _Chaetopterus_ (_after Dahlgren_). _cu_, cuticle; _l. c._, light cells, some showing discharge of secretion; _d. l. c._, discharged and emptied light cells; _m. c._, mucous cells.]

[Illustration: FIG. 21.--Diagram of _Pennatula_, showing by arrows the course of a wave of luminosity which spreads over the colony from the point stimulated (s) (_after Panceri_).]

_Pholas dactylus_ possesses similar light cells to those of _Chaetopterus_, but they are restricted to narrow bands on the siphon and mantle and a pair of triangular spots near the retractor muscles. Nerves pass to the luminous regions.

In many luminous animals the light secretion formed over the surface of the body is small in amount and adheres to the animal because it is embedded in the mucous skin secretions. In those forms which possess a true localized light gland the luminous secretion when expelled into the sea water (if the animal be a marine form) may persist as a luminous streak for some time and exhibit diffusion and convection movements. The most beautiful examples of luminous secretions are found among the ostracod crustacea.

[Illustration: FIG. 22.--Luminous gland of _Cypridina hilgendorfii_ (_after Yatsu_). 2, longitudinal section. 4, transverse section.]

[Illustration: FIG. 23.--Single enlarged gland cell of _Cypridina_ (_after Dahlgren_). P, nucleus and plasmasome; C, cytoplasm; F, secretion fibrils; D, reservoir duct filled with large yellow granules; O, valve-like outer opening of cell at surface of body.]

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, 4-10 mu 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.

The photogenic organ of the firefly is made up of two kinds of cells, a dorsal mass of small cells several layers deep, the reflector layer, and a ventral mass of large cells with indistinct boundaries, the photogenic layer (Fig. 25). The photogenic cells contain a mass of granules, spherical in the male and short rods in the female. The photogenic cells are divided into groups by large tracheal trunks which pass into the light organ and branch to form tracheoles connected with tracheal end cells. The exact distribution varies in different species, but in all the arrangement is such as to give a very abundant oxygen supply. Each group of photogenic cells is surrounded by a clear ectoplasm containing no granules. The tracheoles pass through this and either end openly within the photogenic cells or anastomose with tracheoles from neighboring tracheae. Nerves, but no blood-vessels--which are absent in insects--enter the organ. It is difficult to determine if the nerves supply the tracheal end cells or the photogenic cells.

[Illustration: FIG. 25.--Sectional view of photogenic organ of the firefly (_after Williams_), showing reflector or crystal layer (_U_) above and photogenic cells (_P_) below. _C_, cuticula; _T_, trachea; _c_, capillaries of tracheal end cells; _H_, hypodermis; _EC_, tracheal end cells; _N_, nerve.]

The dorsal reflecting layer is made up of cells containing numerous minute crystals of some purin base, either xanthin or urates, or both. They have a white milky appearance and while they are certainly not good reflectors in the optical sense, they do act as a white background, scatter incident light, and partially prevent its penetration to the internal organs of the firefly. Although a few crystals similar to those of the reflector layer are found in the photogenic cells and in other cells of the body, it is known that the photogenic cells are not transformed into the reflector cells. The two layers are distinct and permanent from an early stage in development.

Curiously enough, the light organ of the larva of the firefly (glowworm) is quite distinct from that of the adult. Like so many other structures in insects, the adult organ is developed anew from potential photogenic cells during the pupal period. Even the egg of the firefly is luminous and glows with a steady light, and during the pupal period light may sometimes be seen coming from the thoracic region.

In the firefly there is no true lens, the light merely shining through the cuticle which is transparent over the light organ, whereas over the rest of the body it is dark and pigmented. In the deep sea shrimp, _Acanthephyra debelis_, with light organs scattered over the surface of the body, the cuticle covering the light organ forms a concavo-convex lens, behind which are the photogenic cells (Kemp, 1910). As may be seen from Fig. 26, the lens is made up of three layers which suggests that it may be corrected for chromatic aberration--a veritable "achromatic triplet." In an allied form, _Sergestes_ (Fig. 27), the lens is of two layers and double convex. Optical studies of these lanterns have been made by Trojan (1907). The course of the light rays is shown in Fig. 28. The lens of these organs is also bluish in color which suggests that they may serve also as color filters. Behind the photogenic cells is a mass of connective tissues through which enters the nerve, for the light of these organs is under the control of the animal and may be flashed "at will."

[Illustration: FIG. 26.--Sectional view of photogenic organ of _Acanthephyra debilis_ (_after Kemp_). _n_, nerve; _s. l._, sheathing layer of cells; _g_, cone of refractive granules at end of nerve strand; _c_, cellular layer; _i. l._, _m. l._, _o. l._, inner, middle and outer layer of lens.]

[Illustration: FIG. 27.--Sectional view of photogenic organ of _Sergestes prehensilis_ (_after Terao_). _bm_, basement membrane; _cs_, connective strands of photogenic layer; _hy_, hypodermis; _l_{1}_, _l_{2}_, _l_{3}_, layers of lens; _le_, lens epithelium; _n_, nerve; _ph_, photogenic cells; _pi_, pigment layer; _r_, reflector; _th_, theca.]

[Illustration: FIG. 28.--Diagram of photogenic organ of _Nyctiphanes Conchii_, to show pathways of light rays arising in the light cell layer (_after Trojan_). _p_, pigment; _ri_, inner reflector; _lp_, light cells; _rf_, refractor; _f_, focus; _l_, lens; _A-A_, axis; _a_{1}-a_{4}_, _b_{1}-b_{4}_, light rays reflected from _ri_; _c_{1}-c_{4}_, light rays passing directly outward; _d_{1}-d_{9}_ and _e_{1}-e_{9}_, light rays which have passed refractor and lens respectively.]

All gradations in complexity of light organs may be found from the condition in the shrimp just described to that found among the squid and fish. Figs. 29 and 30 are sections of two of the more complicated types found in squid. The explanation given to the various structures is that of Chun (1903) to whom we are indebted for a careful histological investigation of these forms. It will be noted that in addition to photogenic and lens tissues there are various types of reflector cells and a line of pigment about the whole inner surface of the organ to effectively screen the animal's tissues from the light. In one form (Fig. 30) chromatophores are found about the region where the light is emitted and these no doubt serve as color filters. There are also an abundant blood supply and nerves passing to the organ. Figs. 30 and 31 are sections through light organs of fishes.

We thus see that light organs may be very simple and also very complicated. The latter must have evolved from the former, although it is not always possible to point out the intermediate stages. It is not within the scope of this book to discuss bioluminescence in its evolutionary aspects. It may be worth while, however, to point out briefly what is known concerning the use of the light to the animal. There are four possibilities.

[Illustration: FIG. 29.--Sectional view of photogenic organ of a squid, _Abraliopsis_ (_after Chun_.) _refl^1_, _refl^2_, reflectors; _lac._, lacunar spaces; _chr._, pigment screen of chromatophores; _chr.^1_, chromatophore; _phot._, photogenic cells; _l_, lens; _co._, cuticle; _v_, blood vessel; _fibr._, connective tissue.]

(1) The light may be of no use whatever, purely fortuitous, an accompaniment of some necessary or even unnecessary chemical reaction.

This appears to be the case in the luminous bacteria and fungi and perhaps the great majority of forms which make up the marine plankton, _Noctiluca_, dinoflagellates, jelly-fish, ctenophores and even the sessile sea pens.

[Illustration: FIG. 30.--Sectional view of photogenic organ of a squid, _Calliteuthis_ (_after Chun_). _phot._, photogenic cells; _l_, _l^1_, lens; _n_, nerve; _spec._, "Spiegel"; _pg._, pigmented screen; _c. fusif._, spindle-shaped reflector cells; _chr._, chromatophore color screen.]

[Illustration: FIG. 31.--Sectional view of photogenic organ of a fish, _Stomias_ (_after Brauer_). _p_, pigment screen; _dr_, _dr^1_, photogenic gland cells; _l_, lens.]

We know that luminous bacteria occasionally lose the power of lighting and that on certain culture media they develop as non-luminous forms. Luminescence is not indispensable to them. The same is true of some of the fungi but _Noctiluca_ and other animals are not known in a non-luminous condition, although we can see no definite value to the organism of this power of luminescence.

[Illustration: FIG. 32.--Sectional view of photogenic organ of a fish, _Argyrophelecus affinis_ (_after Brauer_). _p_, pigmented screen; _dr._, photogenic cells; _r_, _r^1_, reflector?; _l_, lens?; _s_, sclera; _g_, connective tissue.]

In the case of sea pens, however, we might suppose that the light acts as an attraction to small organisms on which the sea pen feeds, although these creatures only luminesce when stimulated in some way, which rather detracts from the above suggestion.

(2) The light may act as a warning to scare away predacious animals which would otherwise feed on the luminous organism. Perhaps this is the case in the sea pens, although these forms possess nematocysts which should serve as adequate protection. The marine worm, _Chaetopterus_, is brightly luminous and lives its whole life in an opaque parchment tube. If this tube were torn open by a predacious form we might conceive that the attacking animal would be alarmed by the light and refrain from destroying the worm. The _Chaetopterus_, however, could not rebuild another tube and its light would only protect it in the night time. These cases will suffice to indicate the difficulties and perplexities of the problem. Perhaps we may add one more guess and suppose that the light of certain fishes is actually for blinding or distracting their enemies or blinding the forms on which they feed. Until this use of luminous organs has actually been observed, we can give little credence to it.

(3) The light may serve as a means of recognition or a sex signal to bring the sexes together for mating. It would seem from the work of Mast and of McDermott that this is the case in the common fireflies and it may be the case in the toad-fish, _Poricthys_, which is only luminous in the spawning season and in the worm, _Odontosyllis_, of Bermuda, which is brilliantly luminous while swarming when the eggs and sperm are shed. It is non-luminous at other times (Galloway and Welch, 1911.)

(4) Finally, it is possible that animals with complex luminous organs, such as squid, fish and shrimp, actually use these as lanterns. It is significant that most of them are deep sea forms, living in a region of perpetual darkness, and it is perfectly logical to suppose that they make use of their light organs for illuminating purposes.

The whole problem of the use and purpose of luminous organs is an exceedingly complex and difficult one. We have, perhaps, said enough to indicate this and may add that in most cases, so far as opinion is based on actual evidence and observation, that of the layman is of as great value as that of the scientist.

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