I THE COLOURS OF ORGANISMS 17 



In the multicellular animals it is curious to note 

 that phosphorescence is most marked in pelagic and 

 in abyssal animals, though also of course occurring 

 elsewhere, e.g. in the glowworm, some centipedes, etc. 



In the Coelentera, many of the Medusae are 

 especially remarkable for their luminosity ; their 

 colouring is thus described by A. Agassiz, "The jelly- 

 fishes, sparkling and brilliant in the sunshine, have 

 a still lovelier light of their own at night. They send 

 out a greenish-golden light, as lustrous as that of 

 the brightest glowworm, and in a calm summer 

 night the water, if you but dip your hand into it, 

 breaks into shining drops beneath your touch." 



As to the remaining groups of the Ccelentera, we 

 find well-marked luminosity in the Siphonophora, in 

 both divisions of the Anthozoa, and in the Ctenophora. 

 In the first group we may specially note Physalia, the 

 Portuguese Man-of-War, which Agassiz describes 

 as appearing like a fire-balloon at night. Of the 

 Anthozoa, the Alcyonaria include the most brilliantly 

 phosphorescent forms ; the Gorgonidae being often 

 described as forming luminous forests at the bottom 

 of the sea, where those of the deep-sea forms which 

 retain their eyes are supposed to congregate. Lumin- 

 ous forms occur also, however, among the Zoantharia, 

 where the deep-sea Sagartia abyssicola, for example, 

 is said to secrete abundant phosphorescent mucus. 



In the Ctenophora phosphorescence is so common 

 as to be practically a class character, and the members 

 of the group are frequently very important in the 

 production of surface phosphorescence. 



In Worms certain of the marine Annelids, such 

 as ChcBtopterus and the Syllidae, are luminous. 



C 



