248 GENERAL ZOOLOGY 



speed. It is interesting to observe them with the help of a 

 pocket flash light on a warm rainy night in spring or early 

 summer. 



When earthworms rest in the daytime, just below the 

 mouth of their burrows, they do so probably as the result 

 of at least four stimuli, — light, heat, fresh air, and mois- 

 ture. If certain of these stimuli were taken away, or even 

 changed in degree, the earthworm would undoubtedly move. 



For example, if the moisture of the ground should become 

 lessened by continued dry weather, the animal would have 

 to forego the fresh air and retreat in order to prevent its skin 

 from drying, which would shut off respiration altogether. If 

 moisture becomes too great, as in a rainstorm, the burrows 

 are filled with water, and most of the oxygen in the narrow, 

 close quarters is driven out and the rest is soon used up. 

 The earthworm is then compelled, if it has been resting near 

 the surface, to leave its burrow and crawl about. This it 

 may do in protected places without serious results, since the 

 heat at such times cannot have its drying effect on the skin. 

 When the storm is over, those that are not caught up by 

 birds or trampled under foot by large animals can make 

 their way into the soil again. 



A food supply is assured to them, for besides the leaves 

 which they drag into their burrows, they devour the soil 

 itself, taking it into their mouths as they burrow along. 

 Black soil called humus is made up of the usual soil min- 

 erals, mixed with a large amount of decaying vegetable 

 matter and some animal matter. Completely decayed or- 

 ganic matter is that which has returned to its original 

 inorganic, mineral state. The partially decayed organic 

 matter can still be used as tissue-building and energy- 

 producing food ; it is this which the earthworms get in 

 the soil they swallow. 



