LUMINESCENCE IN THE SEA 217 



slightest mechanical stimulation is sufficient to start some 

 of the polypes, and the impulse is then communicated to 

 others until every branch and polype is outlined with light 

 Like a series of fairy-lamps. Panceri, who studied the 

 luminosity of many marine animals in the Mediterranean, 

 showed that the luminous matter in Pennatula is produced 

 by eight bands of tissue in the interior of each polype, 

 extending up to papillae surrounding the mouth, so that 

 the secretion was poured out on the surface when lumines- 

 cence took place. The display is, however, in the main, 

 clearly an illumination of the polypes. That is not the 

 case in the closely allied giant sea-pen Funiculina quadrangu- 

 laris (Plate XV, Fig. 1, a dozen specimens about yV ^^^- size), 

 where the colony may attain a length of 5 to 6 feet, 

 and the light is emitted from the mucus on the surface, 

 especially of the axis or stem. I have had both these kinds 

 of sea-pen, freshly dredged in the Hebridean seas, glowing 

 side by side in a tub in the dark on my yacht '' Runa," and 

 in the case of Funiculina, the light, which was of a Hlac 

 colour, compared by Wyville Thomson (Depths of the Sea, 

 p. 149) to the flame of cyanogen gas, came mainly from the 

 surface of the fleshy stem or axis of the colony. The 

 slightest stimulation, such as gentle stroking with the finger, 

 caused great outbursts of light to travel like lambent flames 

 up and down the stem, while the polypes remained com- 

 paratively, if not wholly, in the dark (Plate XV, Fig. 2). 

 G. H. Parker has shown lately that the Alcyonarian colony 

 Renilla, which glows with a beautiful golden green light, 

 spreading over the surface in wave-like ripples from the 

 spot stimulated, can only be excited to luminescence in the 

 night. He was unable to cause any light -production during 

 the day, which suggests that it cannot be wholly a physico- 

 chemical process, but must be in part under nerve-control. 

 In Pennatula and Funiculina, on the other hand, in my 

 experiments on the yacht, I found no difficulty in exciting 

 brilliant luminescence at any hour of the day. 



