234 RESEARCHES ON FUNGI 



away. The actual distance traversed by this slug in the course of its 

 excursion to the two heaps of fungi must have been at least 50 feet. 

 The circumference of a circle with a radius of 21 feet is 132 feet. 

 The width of my heap of Russula nigricans fruit-bodies was 4 inches. 

 Supposing, therefore, that the heap of fruit-bodies were on the 

 circumference of a circle with a radius of 21 feet, as was actually 

 the case in the experiment just recorded, and supposing, further, 

 that a slug were to start from the centre of this circle and move at 

 random radially outwards for a distance of 21 feet, the chances of 

 the slug meeting the heap of fruit-bodies would be 395 to 1 against. 

 Simple mathematical calculations of this kind afford strong evidence 

 that the slugs in my experiments did not find the fungi in the night 

 by chance but through the guidance of some stimulus coming from 

 the fungi and received by their sense organs. 



Immediately after making Experiment VII, I was obliged to 

 leave England to return to duties hi Canada. My investigations 

 upon the finding of fungi by slugs were thereby brought to an end. 

 Slugs and Mustard Gas. There can be but little doubt that the 

 stimulus which comes from the fungi to the slugs and which guides 

 these animals on their foraging expeditions is gaseous in nature. It 

 has been recently shown by Dr. Paul Bartsch of the Smithsonian 

 Institute, Washington, that Limax maximus is extraordinarily 

 sensitive to certain gases. A few years ago a number of slugs of 

 this species, which were under observation in his home, escaped 

 from the box in which they had been confined. Their behaviour 

 in the furnace room showed that they were sensitive to the fumes 

 coming from the furnace and, in response thereto, made character- 

 istic movements of their tentacles. After the United States entered 

 the War and the need for a gas detector arose in connection with 

 the fighting at the front, Dr. Bartsch recalled his furnace-room 

 observations. A very brief period of experimentation then revealed 

 the extraordinary sensitiveness of Limax maximus to mustard gas, 

 and such startling results were obtained that within two hours 

 after the first experiment had been made, the Allied forces were 

 advised by cable of the possibilities of using the slug as a 

 gas detector. Dr. Bartsch found that the tentacles of Limax 

 maximus are sensitive to a dilution of 1 in 10,000,000 of mustard 



