make up the marine plankton, Noctiluca, dinoflagellates, jelly-fish, ctenophores and even the sessile sea pens.
Fig. 30.—Sectional view of photogenic organ of a squid, Calliteuthis (after Chun). phot., photogenic cells; l, l1, lens; n, nerve; spec., "Spiegel"; pg., pigmented screen; c. fusif., spindle-shaped reflector cells; chr., chromatophore color screen.
Fig. 31.—Sectional view of photogenic organ of a fish, Stomias (after Brauer). p, pigment screen; dr, dr1, 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.