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, eg. in the glowworm, some centipedes, etc. 
In the Ccelentera, 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 Chcetopterus and the Syllidse, are luminous. 
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