84 



THE WORM GROUP 



73. Food-taking. — The food of the earthworm is chiefly 

 the soil in which it burrows. By means of an upper lip, 

 which is a specialized anterior segment, and the muscular 

 walls of the pharynx, it takes the earth into its body and 

 the muscles of the digestive tube advance the food along 

 its course. The soluble and therefore digestible parts 

 are absorbed, and the remainder (the greater portion) 



is passed along to the outside. Earth- 

 worms are not critical in the selection of 

 their food, although they are not entirely 

 without a sense of taste. 



74. Economic Importance. — The value 

 of the earthworms to agriculture is too 

 great to be overestimated. In burrow- 

 ing their way through the soil they leave 

 passageways for water and air to enter, 

 thus assisting plants to grow. They 

 bring the fertile, swallowed soil to the 

 surface. When the large numbers of 

 the earthworms are considered, it is 

 obvious that they are the great natural 

 cultivators of the soil. 

 75 Other Annelids. — The sand worm or Nereis (ne're-is), 

 a marine or salt water form, is another segmented annelid. 

 It is more highly specialized than the earthworm, for it has 

 biting mouth parts, tentacles, and eyes. It is an active 

 swimmer at times. The development of the sand worm 

 exhibits metamorphosis, while the earthworm hatches di- 

 rectly into a worm without metamorphosis. 



Figure 82. — Dero. 



A common fresh- 

 water annelid. 



SUMMARY 



In the worm group are included the unsegmented worms, 

 such as tapeworms, liver flukes, and hair worms ; and the 

 segmented or true worms such as the earthworms, sea 



