[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, _Chætopterus_, 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 _Chætopterus_, 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. CHAPTER V THE CHEMISTRY OF LIGHT PRODUCTION, PART I