LUMINOSITY. 265 



the animal is relieved by a dark background : the colour 

 is gradually lost at about a quarter of an inch from its 

 bottom. Then the radiating vessels are of a bright 

 rose-colour, drawn in lines along the colourless surface 

 of -the roof; and the marginal vessel is of the same hue, 

 as are also the four triangular lips of the polypite, with 

 their ciliary fringes. These, as they depend, often ex- 

 tending below the level of the margin, waved about in 

 various directions by the motion of the sea, or by the 

 animal's own movements, add greatly to its elegance. 



That strange and at times magnificent and imposing 

 phenomenon, the luminosity of the sea, is certainly due 

 in part to some of the Medusse. Members of perhaps 

 all the classes of marine invertebrate animals are at one 

 time or other engaged in the illumination, and no doubt 

 the most wide-spread production of spontaneous light, 

 and the most effective, is due to creatures which are 

 individually unrecognisable by the eye. When the ship 

 ploughing through the tropical sea, turns luminous 

 furrows on each side of her prow, and leaves a long 

 wake of curdling light astern, or when the steamer 

 dashes the water of our own estuaries into cascades 

 of fire and showers of coruscating sparks, it is doubt- 

 less to the microscopic Infusoria, Annelida, and Ento- 

 mostraca, that we are mainly indebted for the charming 

 spectacle. Still, many of the Medusse are conspicuously 



