THE WORMS 



215 



Diagrammatic cross section of the body of a cc&- 

 lenterate, and that of a worm. 



get this food. The earth is mixed with fluids poured out from glands in 

 the food tube, and is passed out of the body and deposited on the surface 

 of the ground, in the form of little piles of moist earth. These are familiar 

 sights on all lawns; they are called worm casts. Charles Darwin cal- 

 culated that fifty-three thousand worms may be found in an acre of ground, 

 that ten tons of soil might pass through their bodies in a single year and 

 thus be brought to the surface, and that they plow more soil than all the 

 farmers put together. Earthworms, in spite of their fondness for some 

 garden vegetables and young roots, do much good by breaking up the 

 soU, thus allowing water and oxygen to penetrate to the roots of plants. 



Comparison between Hydra 

 and Worm. — The digesti\e 

 tract of the worm is an almost 

 straight tube inside of another 

 tube. The latter is divided 

 by partitions which mark the 

 boundary of each segment. 

 The outer ca^•ity is known as 

 the body cavity. In the hydra 

 no body cavity exists, there 

 being only a digestive cavity. 

 In the animals higher than the coelenterates the digestive tract and 

 body cavity are distinct. Food is digested within the food tube, is 

 passed through the walls of this tube into the body eavit>', and is in part 

 carried by the blood to various parts of the body. No gills or lungs 

 are present, the thin skin acting as an organ of respiration. But the 

 worm is unable to take in oxygen unless the membranelike skin is kept 

 moist. 



Development. — Notice in some worms the swollen area called the 

 girdle (about one third the distance from the anterior end). This area 

 periodically forms a little sar' in which the eggs of the worm are laid. As 

 it passes toward the anterior end of the worm, it receives from the body 

 openings the sperms and a nutritive fluid in whifh the eggs Ii\e. The 

 fertilized eggs are then left to hatch. The capsules may be found in 

 manure heaps, or under stones, in May or June ; they are small yellow- 

 ish or brown bags about the diameter of a worm. 



Regeneration. — If a one-celled animal be cut into two pieces, each 

 piece, if it contains part of the nucleus, will grow into a whole cell. The 

 hydra, some hydroids, jellyfish, and flatworms, if injured, may grow 

 again parts that are lost. This power is known as regeneration. Earth- 

 worms possess to a large degree the power of replacing parts lost through 

 accident or other means. The anterior end may form a new posterior 

 end, while the posterior end must be cut anterior to the girdle to form 

 a new anterior end. This seems to he in part duo to the greater com- 

 plexity of the organs in the anterior end. 



