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; a1-a4, b1-b4, light rays reflected from ri; c1-c4, light rays passing directly outward; d1-d9 and e1-e9, 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.
Fig. 29.—Sectional view of photogenic organ of a squid, Abraliopsis (after Chun.) refl1, refl2, 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