COLOUR AND PHOSPHORESCENCE 189 



about an hour and then disappearea." In our own seas 

 the most vividly phosphorescent jellyfish is the rounded 

 Pelagia noctilnca (Plate 73) which is however much more 

 abundant in the Mediterranean. The method of light 

 production is probably similar to that of Noctiluca only 

 on a much larger scale, there are no special light organs 

 but a stimulus of any kind, such as that caused by the 

 passage of another animal, causes the whole animal to glow 

 with light. Another animal related to the jellyfishes and 

 even more intensely phosphorescent is the Sea Pen, Pen- 

 natula phosphorea (Plate 73), which lives in mud off the west 

 of Scotland and Scandinavia. When brought to the surface 

 the slightest touch at any part will cause that region to 

 light up in the darkness, the light then spreading from 

 branch to branch until the whole is aglow. The large 

 Sea Pen, Funiculina quadrangular is which may be six 

 feet long, is, according to Professor Herdman, especially 

 phosphorescent along the main stem which, if gently 

 touched, glows with light which travels up and down like 

 a flickering flame. 



The most phosphorescent of the worms is, peculiarly 

 enough, one called Chaetopterus which spends its whole 

 life hidden in a parchment-like tube buried in mud with 

 only the head end protruding (Fig. 39). This animal 

 produces a luminous substance mixed with mucus over 

 almost the entire surface of the body. The light in this 

 case in usually violet or bluish green, and a similar colour 

 is given by many other small marine worms. In some of 

 these, such as one called Odontosyllis from Bermuda, the 

 production of light is concerned with reproduction, light 

 being produced only during the reproductive season when 

 the animals swarm in the sea for mating, and never at other 

 times. 



When we come to the crustaceans we find much more 



