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.
Fig. 22.—Luminous gland of Cypridina hilgendorfii (after Yatsu). 2, longitudinal section. 4, transverse section.