AERATION OF THE SOIL BY EARTHWORMS 399 



less sensitive to external stimulus than usual. This latter 

 conclusion is in accordance with the observations of Darwin 

 (see Vegetable Mould and Earthworms, by Darwin, Chap. I.) 

 who performed some very interesting experiments on worms, 

 undoubtedly Annelids but of undetermined species. In order 

 to verify the statement made by HofTmeister in 1845 that worms 

 are sensitive to the action of light, Darwin exposed some 

 worms, kept in pots, to candle light and observed that usually 

 they were not affected at first. Sometimes, however, they 

 instantly withdrew into their burrows as soon as the light fell 

 on them. This occurred perhaps once out of a dozen times. 

 But all the worms were influenced by the light if exposed to the 

 latter for a sufficient length of time. Darwin's experiments were 

 purely qualitative but apparently receive indirect confirmation 

 from my own quantitative experiments detailed above. 



The Acidity of Worm Castings. — No experiments have 

 been made to determine quantitatively the acidity of worm 

 castings, as this is a subject fraught with peculiar difficulty in 

 view of the fact that the acidity is extremely slight and is 

 readily lost. Thus Darwin mentions that the castings examined 

 by himself, even though clearly acid when first ejected, were 

 perfectly neutral on the succeeding morning. In chalky soil the 

 castings are usually free from all traces of free acid. 



Worms as Aerators of the Soil. — The worms effect the 

 aeration of the soil in a variety of ways, the more important of 

 which may be enumerated as follows : 



1. By burrowing through the soil the worms render the 

 latter more porous and permeable to gases, not merely by virtue 

 of the air spaces formed but by reason of the fact that the soil is 

 thus continually kept in gentle motion. 



2. The soil passing through the bodies of worms is excreted 

 in a finer condition, being ground by attrition within the 

 intestines. Now increased fineness up to a certain limit means 

 increased area and hence greater susceptibility to the action of 

 gases ; beyond this limit the particles begin to cling together 

 and we get a proportionately less porous mass more resembling 

 clay than ordinary soil. Darwin estimates that no fewer than 

 fifteen tons of soil annually pass through the bodies of worms 

 for every acre. 



3. The worms breathe in oxygen and exhale carbon dioxide, 

 which latter, being readily soluble, largely dissolves in water in 



