OXYGEN CONSUMPTION IN ACIDS. 307 



result was reversed, the larger worms exhibiting about 10 per 

 cent, more depression than the small ones. In general, then, the 

 experiments indicate that in concentrations which will kill within 

 a few hours and in which small worms die slightly faster than 

 large worms, the small worms are more greatly depressed by the 

 acid. It should be added that the animals suffered no injury 

 during the period of exposure and recovered completely. 



(&) Previous History with Respect to Acid. From my experi- 

 ments I have gained the distinct impression that the amount of 

 depression induced by exposure to acidified water is to some 

 extent dependent on the previous history with respect to acid. 

 If a given lot of worms, which has never been exposed to acidified 

 water, is tested at rather high acidity, say pH 5.0, it is commonly 

 found that the amount of depression is much less than would 

 ordinarily be expected at that acidity. If on the other hand, 

 such worms are gradually accustomed to acidified water, by 

 exposing them first to pH 7.5 and gradually increasing the 

 acidity, then a much greater percentage of depression is obtained 

 at high acidities. One of the most striking cases of this kind in 

 my investigation occurred in working with sulphuric acid. 

 Three lots of worms, never before exposed to acidified water, 

 were tested in water acidified to pH 5.0 with sulphuric acid. 

 The percentages of depression obtained were 20, 30, and 15 

 respectively. The same lots of worms were then exposed on 

 successive days to various concentrations of sulphuric and 

 hydrochloric acid, beginning at 7.5 and gradually working down 

 to pH 5.0 again. When sulphuric acid pH 5.0 was then again 

 tried, on the same worms, the depressions obtained were respec- 

 tively 39, 56, and 45 per cent. The same type of result was 

 frequently met with. It appears that when worms are suddenly 

 exposed to rather strongly acidified water, they are able to 

 produce or manufacture some substance which protects them 

 from the acid. But after repeated exposures to acidified water, 

 the substance is either exhausted or the worms have become so 

 accustomed to residing in acidified water that they no longer 

 respond to it by producing the substance. Sudden exposure to 

 acid may act as a stimulus to production of basic substances; 



repeated exposure fails to stimulate. 

 22 



