156 DIVERSIONS OF A NATURALIST 



I have mentioned these phosphorescent organs of 

 small and smallest crustaceans because not many years 

 ago a French naturalist, my friend Professor Giard, 

 found that many of the sand-hoppers on the great sandy 

 shore near Boulogne are phosphorescent. A year or 

 two later I found them myself on the shore above tide^ 

 mark at Ouistreham (Westerham), near Caen, where 

 they had actually been mistaken for glow-worms ! It 

 was easy at night to pick up a dozen phosphorescent 

 sand-hoppers during a stroll of five or ten minutes on 

 the sands. Yet I have never seen them nor heard of 

 their being seen on the English coast, and one of the 

 results which I hope for in mentioning them here is that 

 some of my readers will discover them on British sands 

 and let me know. The remarkable fact about the 

 luminous sand-hoppers is that they have no apparatus 

 for producing light, and, as a matter of fact, do not pro- 

 duce it ! Their luminosity is a disease, and is due (as 

 was shown by that much-beloved teacher and dis- 

 coverer the late Professor Giard) to the infection of their 

 blood by a bacillus. Hence it is only here and there 

 that you see the brilliant greenish ball of light on the 

 sand due to a phosphorescent sand-hopper. And when 

 you pick it up you find that the poor little thing is quite 

 feeble and unable to hop. Examine its blood under the 

 microscope and you find it teeming with excessively 

 minute parasitic rods like those which cause the phos- 

 phorescence of dead fish, of stale bones, and occasionally 

 of butcher's meat. Similar bacilli may be obtained by 

 cultivation from any sea-water, and in such abundance 

 that a room can be lit up by a bottleful of the cultiva- 

 tion. Perhaps all the light-producing bacteria or bacilli 

 are only varieties of one species — perhaps they are dis- 

 tinct species. Whether a species or a variety, that 

 which gets into the blood of the sand-hopper and 



