The _medusæ_ differ in their methods of illumination. The _Obelia_, as a free-swimming disk, is non-luminous; but the stem, or trophosome, out of which it is developed, has a fluctuating light extending up and down its surface. In many _medusæ_ the light appears to be confined to the upper portion of the umbrella, to the tentacles, and to the margin of the disk; but if an oar is thrust through it, or a freshly stranded jelly is torn and cut upon the sand, every portion seems to become more or less luminous.[8] The little jelly-like creatures called “comb-bearers,” or Ctenophores, are nearly all wonderfully phosphorescent. Instead of moving as do the ordinary jelly-fishes, they have rows of comb-like paddles which move up and down in regular measure as they float along. In the daytime the little fins gleam with gorgeous iridescent hues; while at night they are brilliantly luminous, even the eggs and embryos of some emitting light. The _Beroë_ (Plate IV., Fig. 1) is the most familiar, but the Pleurobrachia is the most graceful. Drummond refers to these forms in the following lines,-- “Shaped as bard’s fancy shapes the small balloon, To bear some sylph or fay beyond the moon. From all her bands see lurid fringes play, That glance and sparkle in the solar ray With iridescent hues. Now round and round She whirls and twirls; now mounts, then sinks profound.” [Illustration: PLATE V. VENUS’ GIRDLE. (_Cestus veneris._) _Phillerhoe._] So vast are the numbers of these and other light-givers in the northern seas, that the olive-green tints of the waters are due to them in the daytime. Mr. Scoresby, finding sixty-five of them in a cubic inch of water, summed up the interesting calculation, that, if eighty thousand persons had commenced at the beginning of the world (he refers to the popular, not geological, reckoning,) to count, they would barely at the present time have completed the enumeration of individuals of a single species found in a cubical mile.