THE EARTHWORM 219 



Earthworms are sensitive to changes in temperature, al- 

 though when in otherwise favorable situations they will 

 endure a rise from 18 C. to 28 C. (64.4 F. to 82.4 F.) with- 

 out changing their position, a fact determined by some 

 experiments carried on at Bryn Mawr College. When earth- 

 worms 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 moisture. 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 rain-storm, 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. When earthworms are engaged in 

 boring through the soil, the air in their burrows is in circula- 

 tion, and even though the amount of oxygen may be small, 

 it appears to be sufficient to release the necessary amount of 

 energy for their work. 



When animals become adapted in structure and habit to 

 their environment, the result is a definite food-supply and 

 protection to themselves and their race. Certainly the food- 

 supply of earthworms is assured to them, for besides the leaves 

 which they drag into their burrows, they are known to devour 

 the soil itself, taking it into their mouths as they burrow 



