48 ANIMAL BIOLOGY 



inner, thicker layer of fibers which run lengthwise. The 

 worm crawls by shortening the longitudinal muscles. As 

 the bristles (setce) point backward, they prevent the front 

 part of the body from slipping back, so the hinder part is 

 drawn forward. Next, the circular muscles contract, and 

 the bristles preventing the hind part from slipping back, 

 the fore portion is pushed forward. Is the worm thicker 

 when the hinder part is being pulled up or when the fore 

 part is being thrust forward ? Does the earthworm pull or 

 push itself along, or does it do both ? Occasionally it trav- 

 els backward, e.g. it sometimes goes backward into its hole. 

 Then the bristles are directed forward. 



The right and left halves of the body are counterparts of 

 each other, hence the earthworm is bilaterally symmetrical. 

 The lungs and gills of animals must always be kept moist. 

 The worm cannot live long in dry air, for respiration in the 

 skin ceases when it cannot be kept moist, and the worm 

 smothers. Long immersion in water is injurious to it, 

 perhaps because there is far less oxygen in water than in 

 the air. 



Darwin wrote a book called "Vegetable Mold and Earth- 

 worms." He estimated that there were fifty thousand earth- 

 worms to the acre on farm land in England, and that they 

 bring up eighteen tons of soil in an acre each year. As 

 the acids of the food tube act upon the mineral grains that 

 pass through it, the earthworm renders great aid in form- 

 ing soil. By burrowing it makes the soil more porous and 

 brings up the subsoil. 



Although without eyes, the worm is sensitive to light 

 falling upon its anterior segments. When the light of a 

 lantern suddenly strikes it at night, it crawls quickly to its 

 burrow. Its sense of touch is so keen that it can detect a 

 light puff of breath. Which of the foods kept in a box of 



