Part 6 (1/2)

[Ill.u.s.tration: FIG. 27.--Sectional view of photogenic organ of _Sergestes prehensilis_ (_after Terao_). _bm_, bas.e.m.e.nt 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.]

[Ill.u.s.tration: 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 pa.s.sing directly outward; _d_{1}-d_{9}_ and _e_{1}-e_{9}_, light rays which have pa.s.sed 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) chromatoph.o.r.es 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 pa.s.sing 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.

[Ill.u.s.tration: FIG. 29.--Sectional view of photogenic organ of a squid, _Abraliopsis_ (_after Chun_.) _refl^1_, _refl^2_, reflectors; _lac._, lacunar s.p.a.ces; _chr._, pigment screen of chromatoph.o.r.es; _chr.^1_, chromatoph.o.r.e; _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, ctenoph.o.r.es and even the sessile sea pens.

[Ill.u.s.tration: 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._, chromatoph.o.r.e color screen.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 s.e.x signal to bring the s.e.xes 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 sp.a.w.ning 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.

CHAPTER V

THE CHEMISTRY OF LIGHT PRODUCTION, PART I

Two experiments, both performed very early in the history of Bioluminescence, are of great importance in understanding the nature of animal light. Boyle (1667), as already mentioned, proved the necessity of air for the luminescence of wood and fish and Spallanzani (1794) showed that parts of luminous medusae gave no light when dried but if moistened again would emit light as before. We see then, that air (oxygen), water, and some photogenic substance are necessary for the light production. Spallanzani's experiment, which has been confirmed for a great many luminous forms, shows also that animal luminescence is not a _vital_ process, in the same sense that the conduction of a nerve impulse is a vital process. A nerve loses its characteristic property of conduction on drying or maceration while luminous cells still possess the power to luminesce after drying or maceration. Using the terminology of the older physiology we may say that ”living protoplasm” is not necessary for light production.

The experiments of Boyle (1626-91) are of great interest, especially those in which he studied the behavior of s.h.i.+ning wood under the receiver of his air pump. On October 29, 1667, he wrote:

”Exp. I.: Having procured a Piece of _s.h.i.+ning Wood_, about the bigness of a groat or less, that gave a vivid Light, (for _rotten Wood_) we put it into a middle sized _Receiver_, so as it was kept from touching the Cement; and the _Pump_ being set a-work, we observed not, during the 5 or 6 first Exsuctions of the Air, that the splendor of the included Wood was manifestly lessened (though it was never at all increased;) but about the 7th Suck, it seemed to glow a little more dim, and afterwards answered our Expectation, by losing of its Light more and more, as the Air was still farther pumped out; till at length about the 10th Exsuction, (though by the removal of the Candles out of the Room, and by black Cloaths and Hats we made the place as dark as we could, yet) we could not perceive any light at all to proceed from the _Wood_.