COLOUR AND PHOSPHORESCENCE 193 



transparent mass of small animals which unite one with 

 another to form a hollow cylinder one end of which is 

 closed, is found drifting about near the surface of the sea 

 and forms a conspicuous member of the plankton in such 

 regions as the Mediterranean and tropical seas. Each 

 member of the colony has two light-producing glands and 

 the light they give out is said to be red in some and blue in 

 others, the whole colony when stimulated blazing with 

 thousands of these tiny points of light. Moseley, one of 

 the naturalists on the Challenger, states that, after a 

 Pyrosoma more than four feet long had been captured, 

 " I wrote my name with my finger on the surface of the 

 giant Pyrosoma as it lay on deck in a tub at night, and my 

 name came out in a few seconds in letters of fire." 



At one time it was thought that luminous fishes were 

 especially common in the deepest seas, regions of utter 

 darkness, the light being used to assist them in their 

 search for food. This is now known to be incorrect for, 

 though the fish living on the surface, such as the herring 

 or mackerel, never display luminescence, yet this power is 

 commonest in fish living at moderate depths, on the bottom 

 or in the intermediate regions. Light organs are apparently 

 commonest in fish which live in the upper 500 metres of 

 the warmer seas, although there are notable exceptions, 

 for example, a remarkable fish called Harpodon which 

 lives in rivers and estuaries in India and, when caught, 

 displays the most vivid phosphorescence, and another 

 called Photoblepharon found in pools of fresh water in 

 quarries and the craters of extinct volcanoes in Malaya. 

 The actual organs vary a great deal in both structure and 

 position. They may be mere pits and channels for the 

 production of a luminous slime, such as are found in some 

 of the deep-sea Macruridae, or they may be complex organs 

 like the natural lamps we have described in some of the 



