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.