LUMINESCENCE 



569 



movement. In Pelagia noctiluca luminescence appears on the bell and 

 tentacles in response to tactile stimulation. A slight touch on the outer 

 surface of the umbrella results in a flash of light at the point of contact, 

 and the light then spreads but may not cover the whole surface of the 

 bell. Under strong tactile stimulation the light is brighter and spreads 

 over the entire umbrella surface. 



In various hydroids such as Campanularia and in sea-pens (Pennatu- 

 lacea), a local stimulus gives rise to a wave of luminescence. Special atten- 

 tion has been paid to the luminous responses of sea-pens {Renilla, Cavernu- 

 laria, Pennatula, etc.). In these animals the whole colony may be lumines- 

 cent {Cave rnul ar id), or only the polyps {Pennatula, Renilla). Luminescence 



Fig. 13.25. Diagrammatic Representation of the Directions taken by 

 Luminous Waves in Pennatula, following Tactile Stimulation 



S indicates the point of stimulation; and the arrows, the course of the luminous 

 waves which travel over the animal. (From Panceri (51).) 



is intracellular, and appears as a wave of light which proceeds from the 

 point of stimulation over the surface of the colony. The wave results from 

 the lighting-up of zooids in regular succession. Characteristics of the 

 luminous flashes of sea-pens and some other luminous marine animals are 

 given in Table 13.3. 



Transmission of the luminescent response in sea-pens takes place in 

 a non-polarized nerve-net, and the excitation can proceed in any direction. 

 Thus, stimulation at the base of a specimen gives rise to luminescent 

 waves which ascend to the tip, and stimulation of the tip results in waves 

 which run to the base (Fig. 13.25). When the centre of the stalk or a side 

 branch is touched, luminescent waves proceed in both directions, up and 

 down the stalk. Simultaneous stimulation of the base and tip of the colony 

 gives rise to waves which meet at the centre and extinguish each other. 

 It is furthermore observed that the luminous waves are not stopped by 



