crystal transformation are not very convincing. Pierantoni (1915) has considered the granules to be symbiotic luminous bacteria, but this is certainly not the case.
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.
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.