548 



THE BIOLOGY OF MARINE ANIMALS 



groove and then spreads as a wave over the surface of the animal. The 

 actual spots of illumination are stationary, and they flash on and off in 

 succession as the wave of illumination passes over the cell. Luminescence 

 is also intracellular in radiolarians, where it appears as a weak and diffuse 

 bluish light following stimulation. 



In ctenophores light is produced by glandular structures lying within the 

 eight radial canals. These appear as brilliant greenish streaks when the 



Fig. 13.9. A Ctenophore Pleurobrachia pileus, Showing the Appearance of a 

 Lighted Animal (from Dahlgren, 1916.) 



animal is stimulated (Fig. 13.9). The photogenic cells are granular elements 

 lying in the outer walls of the gastro-vascular canals. There is some doubt 

 whether luminescence is intracellular in this group, or whether a luminous 

 secretion is released into the vascular canals. It has been noted that if 

 specimens are squeezed through cheese cloth they yield a crude luminous 

 extract similar to that of medusae. Dahlgren, however, could detect no 

 secretory discharge in the canals and concluded that the light was pro- 

 duced within the cell. 



