74 THE NATURE OF ANIMAL LIGHT 



crystal transformation are not very convincing. Pieran- 

 toni (1915) has considered the granules to be symbiotic 

 luminous bacteria, but this is certainly not the case. 



The light of Chcetopterns comes from a material mixed 

 with a mucous secretion formed over almost the whole 

 body surfaces of the animal. A secretion of the epithe- 

 lium 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. 



Pholas dactylus possesses similar light cells to those 

 of Chcetopterus, 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 lumi- 

 nous 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 



