_symbiotic_ luminous bacteria, but this is certainly not the case. [Illustration: FIG. 19.--Diagram of _Pholas_ (right) and _Chætopterus_ (left) to show distribution of luminous areas (_after Panceri_).] The light of _Chætopterus_ comes from a material mixed with a mucous secretion formed over almost the whole body surfaces of the animal. A section of the epithelium shows large mucous-producing cells and smaller granule-containing light cells (Fig. 20). These appear to be under nervous control, as a strong stimulation in one part of the body causes luminescence which spreads over the whole surface of the worm. The animal becomes fatigued rather readily, however. In the pennatulids, such as _Cavernularia_, we have also the formation of a luminous secretion over the whole surface of the body and the individual animals in this colonial form are also connected with nerves. A stimulation in any local region, as Panceri (1872) first showed (Fig. 21), will cause a wave of luminosity to spread from this point until it extends over the whole surface of the colony. In _Pennatula_ the rate of this luminous wave is about 5 cm. per second. [Illustration: FIG. 20.--Sectional view of the luminous epithelium of _Chætopterus_ (_after Dahlgren_). _cu_, cuticle; _l. c._, light cells, some showing discharge of secretion; _d. l. c._, discharged and emptied light cells; _m. c._, mucous cells.] [Illustration: FIG. 21.--Diagram of _Pennatula_, showing by arrows the course of a wave of luminosity which spreads over the colony from the point stimulated (s) (_after Panceri_).] _Pholas dactylus_ possesses similar light cells to those of _Chætopterus_, but they are restricted to narrow bands on the siphon and mantle and a pair of triangular spots near the retractor muscles. Nerves pass to the luminous regions. In many luminous animals the light secretion formed over the surface of the body is small in amount and adheres to the animal because it is embedded in the mucous skin secretions. In those forms which possess a true localized light gland the luminous secretion when expelled into the sea water (if the animal be a marine form) may persist as a luminous streak for some time and exhibit diffusion and convection movements. The most beautiful examples of luminous secretions are found among the ostracod crustacea. [Illustration: FIG. 22.--Luminous gland of _Cypridina hilgendorfii_ (_after Yatsu_). 2, longitudinal section. 4, transverse section.] [Illustration: FIG. 23.--Single enlarged gland cell of _Cypridina_ (_after Dahlgren_). P, nucleus and plasmasome; C, cytoplasm; F, secretion fibrils; D, reservoir duct filled with large yellow granules; O, valve-like outer opening of cell at surface of body.]