Phylum Annelida and Other Wonnlike Coelomates 461 



tive reaction to light and heat and a positive one to moisture. Mechani- 

 cal stimulation evokes a response from the earthworm : they are strongly 

 thigmotactic. Strong vibratory stimuli result in the withdrawal of the 

 worm into its burrow. 



From these responses, it can be easily understood that the earth- 

 worms are nocturnal animals which remain hidden in their burrows 

 during the daytime. They emerge only at night or after a heavy rain. 

 The burrows offer great protection to the earthw-orm ; they are able to re- 

 main securely in them by means of their setae. Often the w^orm extends 

 the anterior portion of the body out from the burrow, leaving the pos- 

 terior part firmly anchored. This allows for rapid retreat in case of 

 necessity. In light soils, the earthworm simply pushes its way through, 

 while in heavier ones, they literally eat their way through, casting the 

 mounds of feces at the surface of the burrow. They feed on organic 

 detritus such as leaf mold which they suck into their buccal cavity by 

 means of the muscular pharynx. 



Compared to the fiatworms, the earthworm has but limited powers 

 of regeneration. The adult, however, can regenerate lost tail segments 

 or even head segments. 



Earthworms exist in great numbers in such places as old pastures, 

 lawns, and gardens. Charles Darwin estimated that about 26,886 worms 

 could live in an acre of old pasture. This many earthworms play an 

 amazing part in turning over and aerating the soil. Since they make 

 burrows in the soil by literally eating their way through, they bring the 

 lower soil to the surface in their castings. Again Darwin estimated that 

 if each worm annually ejected 20 ounces of earth, it would make a 

 total of nearly 15 tons thrown up annually by the acre containing 26,886 

 w'orms. From his observations, he felt that the activity of worms played 

 a great part in the burying of heavy objects in fields. Since earthworms 

 feed on organic materials such as dead leaves which they drag into their 

 burrows, they play a great part in fertilizing the soil. 



The earthworms play a minor role as vectors of animal diseases. 

 They are intermediate hosts for fowl cestode and pig lungworm and 

 carriers for gapeworm of chickens. 



Other Oligochaetes. — Compared to the polychaetes, the oli- 

 gochaetes are a highly uniform group so far as appearance is concerned. 

 A number of species of earthworms occur, but all bear close resemblance 

 to one another. In fresh water, many small oligochaetes occur, includ- 

 ing such genera as Chaetogaster, Aeolosoma, Nais, and Stylaria. In 



